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, 46 insertions, 0 deletions

diff --git a/test/snapshots/multicursor.snap b/test/snapshots/multicursor.snap
new file mode 100644
index 00000000..2bfccb93
--- /dev/null
+++ b/test/snapshots/multicursor.snap
@@ -0,0 +1,46 @@
+# MULTIPLE CURSORS on a real file pane. The differential suite (hxdiff) proves
+# the model against a real helix but is headless, so this is the part it cannot
+# see: that every range is actually on screen, that the primary reads
+# differently from the others, and that ONE keystroke edits at all of them.
+lines f.txt 12
+start 30 150 -n 3
+wait 8000 f.txt
+stable 700 20000
+# open f.txt in the left pane (type its name, right-click to Look)
+press left 5 8
+release left 5 8
+stable 400 5000
+key i
+text f.txt
+key esc
+settle 100
+press right 6 8
+release right 6 8
+wait 10000 line 3
+stable 700 15000
+# top of the file, then C C: three cursors, one per line, primary on line 3
+key g g
+stable 400 5000
+key C C
+stable 400 5000
+snap three-cursors
+# the colours ARE the feature here: the primary's cell against the two dimmer
+# ones, each with its own bright cursor cell
+snapstyle three-cursors
+# one keystroke, three edits
+key i
+text >>
+key esc
+settle 100
+stable 400 5000
+snap edited-at-three
+# a selection per cursor: e selects to the end of the first word at each
+key g h w
+stable 400 5000
+snap three-selections
+snapstyle three-selections
+# `,` drops back to the primary alone; the two dim highlights must vanish
+key ,
+stable 400 5000
+snap keep-primary
+snapstyle keep-primary