multiple cursors, regex selection, and Ctrl-c comments

The primary cursor stays exactly where it was — cur_row/cur_col plus vsel — and
sels[] holds helix's OTHER ranges. That split is why nothing moved at one
cursor: with nsel == 0 not one line of the existing motion, operator, render or
mouse code takes a different branch, which is what protects 800 differential
cases and 67 goldens.

paneRanges/setPaneRanges are the whole list; setPaneRanges IS helix's
Selection::new (min width 1, sorted, overlaps merged, primary follows its range
through a merge). An ordinary key runs the single-selection handler once per
range, visited last-first so an edit never disturbs a range still waiting, and
each finished pass is remembered as a distance from the END of the text, which
an earlier edit cannot move — helix's change mapping without a change map.
pushUndo fires once per keystroke, yanks accumulate, and a builtin acts from
the primary and stops the replay, which also closes the use-after-free window
if it frees the pane.

s and S reuse the / prompt wholesale rather than growing a second one: the
pattern is typed into the tag tail, and every keystroke re-runs the match from
the selection the prompt opened on, so the preview is live and Esc is just the
empty pattern. mvzr does runtime patterns — a bytecode VM in a fixed-size
struct with no allocator — with 64 ops and 8 char classes per pattern, no
case-insensitive flag (helix's smart case is done by folding a scratch copy),
no captures, no multi-line anchors. The last two are the two waivers.

Ctrl-c is a whole-list key and not a per-cursor replay, because helix decides
comment-vs-uncomment ONCE for the whole selection; replaying it would take that
decision n times. Comment tokens are a table in config.zig keyed on the same
extension syntax.zig picks grammars by.

Found and fixed a pre-existing single-cursor bug on the way: la<bs><esc> left
the cursor one cell before where the append began. helix's restore_cursor can
never walk past the origin; ours backed up unconditionally. hxdiff was green
before AND after — the old one-selection contract could not see it.

hxdiff 360 -> 481 cases, hxparity 440 -> 561, all goldens from real helix; the
harness contract now reports every range and its primary, omitted when there is
one, so 359 of the 360 old goldens are byte-identical. The one that moved is
o-count: helix's 2o really does leave two cursors and could not say so before.

1 files changed, 275 insertions, 0 deletions

diff --git a/test/snapshots/selregex.golden b/test/snapshots/selregex.golden
new file mode 100644
index 00000000..9a1d58d6
--- /dev/null
+++ b/test/snapshots/selregex.golden
@@ -0,0 +1,275 @@
+== snap prompt-armed grid=160x24 cursor=59,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  INS /tmp/pardes-snap/selregex/cwd/m.zig Save Del Select /                       NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 const one = 1;
+|     2 const two = 2;                                                             m.zig
+|     3 const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== snap preview grid=160x24 cursor=64,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  INS /tmp/pardes-snap/selregex/cwd/m.zig Save Del Select /const                  NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 const one = 1;
+|     2 const two = 2;                                                             m.zig
+|     3 const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== style preview
+|0: 0-52 #949494,#121212, 53-159 d,#121212,
+|1: 0-1 d,#626262, 2-63 #949494,#121212, 64-79 d,#121212, 80-81 d,#262626, 82-118 #949494,#121212, 119-159 d,#121212,
+|2: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #d8c8a8,#4a3a1e, 11-11 #4a3a1e,#d8c8a8, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|3: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #a89c84,#2e2616, 11-11 #4a3a1e,#d8c8a8, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|4: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #a89c84,#2e2616, 11-11 #4a3a1e,#d8c8a8, 12-20 #bdbdbd,#080808, 21-21 #e3c78a,#080808, 22-22 #bdbdbd,#080808, 23-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|5: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|6: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|7: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|8: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|9: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|10: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|11: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|12: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|13: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|14: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|15: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|16: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|17: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|18: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|19: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|20: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|21: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|22: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|23: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+== snap three-selected grid=160x24 cursor=11,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/selregex/cwd/m.zig Save Del                                NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 const one = 1;
+|     2 const two = 2;                                                             m.zig
+|     3 const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== style three-selected
+|0: 0-52 #949494,#121212, 53-159 d,#121212,
+|1: 0-1 d,#626262, 2-49 #949494,#121212, 50-79 d,#121212, 80-81 d,#262626, 82-118 #949494,#121212, 119-159 d,#121212,
+|2: 0-1 d,#444444, 2-6 #949494,#121212, 7-11 #d8c8a8,#4a3a1e, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|3: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #a89c84,#2e2616, 11-11 #4a3a1e,#d8c8a8, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|4: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #a89c84,#2e2616, 11-11 #4a3a1e,#d8c8a8, 12-20 #bdbdbd,#080808, 21-21 #e3c78a,#080808, 22-22 #bdbdbd,#080808, 23-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|5: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|6: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|7: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|8: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|9: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|10: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|11: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|12: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|13: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|14: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|15: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|16: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|17: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|18: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|19: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|20: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|21: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|22: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|23: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+== snap narrowed grid=160x24 cursor=60,1
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  INS /tmp/pardes-snap/selregex/cwd/m.zig Save Del Select /t                      NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 const one = 1;
+|     2 const two = 2;                                                             m.zig
+|     3 const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== style narrowed
+|0: 0-52 #949494,#121212, 53-159 d,#121212,
+|1: 0-1 d,#626262, 2-59 #949494,#121212, 60-79 d,#121212, 80-81 d,#262626, 82-118 #949494,#121212, 119-159 d,#121212,
+|2: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-11 #4a3a1e,#d8c8a8, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|3: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-11 #4a3a1e,#d8c8a8, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|4: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #bdbdbd,#080808, 11-11 #4a3a1e,#d8c8a8, 12-20 #bdbdbd,#080808, 21-21 #e3c78a,#080808, 22-22 #bdbdbd,#080808, 23-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|5: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|6: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|7: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|8: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|9: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|10: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|11: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|12: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|13: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|14: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|15: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|16: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|17: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|18: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|19: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|20: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|21: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|22: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|23: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+== snap esc-restores grid=160x24 cursor=11,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/selregex/cwd/m.zig Save Del                                NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 const one = 1;
+|     2 const two = 2;                                                             m.zig
+|     3 const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== style esc-restores
+|0: 0-52 #949494,#121212, 53-159 d,#121212,
+|1: 0-1 d,#626262, 2-49 #949494,#121212, 50-79 d,#121212, 80-81 d,#262626, 82-118 #949494,#121212, 119-159 d,#121212,
+|2: 0-1 d,#444444, 2-6 #949494,#121212, 7-11 #d8c8a8,#4a3a1e, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|3: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #a89c84,#2e2616, 11-11 #4a3a1e,#d8c8a8, 12-18 #bdbdbd,#080808, 19-19 #e3c78a,#080808, 20-20 #bdbdbd,#080808, 21-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|4: 0-1 d,#444444, 2-6 #626262,#080808, 7-10 #a89c84,#2e2616, 11-11 #4a3a1e,#d8c8a8, 12-20 #bdbdbd,#080808, 21-21 #e3c78a,#080808, 22-22 #bdbdbd,#080808, 23-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|5: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|6: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|7: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|8: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|9: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|10: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|11: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|12: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|13: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|14: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|15: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|16: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|17: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|18: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|19: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|20: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|21: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|22: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+|23: 0-1 d,#444444, 2-6 #626262,#080808, 7-79 d,#080808, 80-81 d,#444444, 82-159 #bdbdbd,#080808,
+== snap commented grid=160x24 cursor=14,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/selregex/cwd/m.zig Save Del                                NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 // const one = 1;
+|     2 // const two = 2;                                                          m.zig
+|     3 // const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+== snap uncommented grid=160x24 cursor=11,2
+|Kill Newcol Tutor Debug NextColor Dump Find Grep Help
+|  NOR /tmp/pardes-snap/selregex/cwd/m.zig Save Del                                NOR /tmp/pardes-snap/selregex/cwd Del
+|     1 const one = 1;
+|     2 const two = 2;                                                             m.zig
+|     3 const three = 3;
+|     4
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|
+|