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, 56 insertions, 0 deletions

diff --git a/test/snapshots/selregex.snap b/test/snapshots/selregex.snap
new file mode 100644
index 00000000..193ab314
--- /dev/null
+++ b/test/snapshots/selregex.snap
@@ -0,0 +1,56 @@
+# Interactive regex selection (`s` / `S`) and the comment toggle, on a real
+# file pane. hxdiff pins what they DO against a real helix, but it is headless,
+# so this is the half it cannot see: the prompt living in the TAG rather than a
+# popup, the matches lighting up under a pattern as it is typed, Esc putting
+# back what was there, and Ctrl-c using the token this file's EXTENSION asks
+# for — `//`, where a buffer with no language would get `#`.
+file m.zig const one = 1;\nconst two = 2;\nconst three = 3;\n
+# wide on purpose: the whole point of this script is a tag long enough to hold
+# the marker AND the pattern, and a narrow pane simply truncates it away
+start 24 160 m.zig
+wait 8000 m.zig
+stable 700 20000
+key g g
+stable 400 5000
+# `%` takes the whole file, `s` arms the regex input: the marker goes into the
+# tag tail after the path, and the pane is in insert mode ON THE TAG — the
+# same one-line editor `/` and Find use, and nothing has moved in the body yet.
+key %
+key s
+stable 400 5000
+snap prompt-armed
+# ...and now the point: the selection follows the pattern as it is typed. Three
+# `const` runs light up with Enter nowhere near, and the colours ARE the
+# feature — the primary bright, the other two dim, each with its own cursor.
+text const
+stable 400 5000
+snap preview
+snapstyle preview
+key enter
+stable 400 5000
+snap three-selected
+snapstyle three-selected
+# a second `s` narrows the three to one `t` apiece...
+key s
+text t
+stable 400 5000
+snap narrowed
+# the grid text cannot show a selection, so this is where the styles carry the
+# whole point: three one-cell ranges here, three five-cell ones after the Esc
+snapstyle narrowed
+# ...and Esc throws that away for the selection the prompt opened on, which is
+# free: the preview is re-derived from that snapshot on every keystroke, so
+# cancelling is just applying the empty pattern.
+key esc
+settle 100
+stable 400 5000
+snap esc-restores
+snapstyle esc-restores
+# Ctrl-c comments every line the three cursors touch — one decision for the
+# whole selection, not one per cursor — and again to take it back off.
+key c-c
+stable 400 5000
+snap commented
+key c-c
+stable 400 5000
+snap uncommented