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, 7 insertions, 7 deletions

diff --git a/test/snapshots/lspdebug.golden b/test/snapshots/lspdebug.golden
index 2bce4aa6..bb0fee41 100644
--- a/test/snapshots/lspdebug.golden
+++ b/test/snapshots/lspdebug.golden
@@ -203,16 +203,16 @@
 |     3 zig lib dir: /usr/lib/zig  [OK]
 |     4 offsets: utf-8   walk caps: 512 files, 2000 rows
 |     5
-|     6 dependency imports gd can follow (6):
+|     6 dependency imports gd can follow (7):
 |     7   zls            /home/goblin/00-projects/0x4200.cafe
 |     8   tree-sitter    /home/goblin/00-projects/0x4200.cafe
 |     9   zstbi          /home/goblin/00-projects/0x4200.cafe
-|    10   ghostty-vt     /home/goblin/00-projects/0x4200.cafe
-|    11   vaxis          /home/goblin/00-projects/0x4200.cafe
-|    12   uucode         /home/goblin/00-projects/0x4200.cafe
-|    13 asked from: /tmp/pardes-snap/lspdebug/cwd/z.zig
-|    14
-|    15 answers: definition, declaration, type_definition, im
+|    10   mvzr           /home/goblin/00-projects/0x4200.cafe
+|    11   ghostty-vt     /home/goblin/00-projects/0x4200.cafe
+|    12   vaxis          /home/goblin/00-projects/0x4200.cafe
+|    13   uucode         /home/goblin/00-projects/0x4200.cafe
+|    14 asked from: /tmp/pardes-snap/lspdebug/cwd/z.zig
+|    15
 |  NOR /tmp/pardes-snap/lspdebug/cwd/+Lsp Del
 |     1 lsp explain — the definition query at byte 29 of z.zi
 |     2