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, 1 insertions, 1 deletions

diff --git a/next-steps.txt b/next-steps.txt
index b421f7a4..7599ebdd 100644
--- a/next-steps.txt
+++ b/next-steps.txt
@@ -1,7 +1,7 @@
 + All syntatic choices must be centralized in single .zig file: keybindings, syntax for Look, etc; the motivation for this is that those can easilly be changed and tweaked if they're centralized like this and in the process we may cut down some repetition of logic.
 + Carve out and isolate the Output pane as its own semantic entity, right now we are doing specific stuff on the output pane based on the command that opened it, let's make this part of the data structure that represents it, the heuristics for special handling of the output panes will be done by reading the Command that originated it field from itself. Make this centralized on the code so its clear to see and tweak those heuristics.
 + File panes need to split the latest column which a file pane was being used.
-- Add multiple cursor support, and Ctrl-c auto comments. Use the same methodology of running helix in a harness as the oracle for differential testing. Also add regex interactive selection like on helix and the more specific selection semantics, use https://github.com/mnemnion/mvzr for regexes, add it using zig fetch.
++ Add multiple cursor support, and Ctrl-c auto comments. Use the same methodology of running helix in a harness as the oracle for differential testing. Also add regex interactive selection like on helix and the more specific selection semantics, use https://github.com/mnemnion/mvzr for regexes, add it using zig fetch.
 + Add a new builtin that will list all builtins and their respective shortcuts if they have any (note that shortcuts may be spc+... or just keyboard or mouse chords/movements)
 + Scrolling on a touchpad moves horizontally accidentally way too much, add some heuristics to prioritize vertical scrolling and make horizontal scrolling work only when the user is a bit more explicit about it - for example by measuring if there was recent high-enough vertical movement, etc.
 + Add a new builtin to list the themes and create an output window to select those themes; each theme now must live on its own .zig file, when moving with n/N on out ThemeSel output window it will select that theme so it's a interactive way to select the themes. We're being clever now with how these output windows will work, this feature depends on the new semantics for output windows but basically now the ThemeSel output panes will set the Command flag that will basically tell that when doing n/N movement on the output pane instead of Look-ing it will Execute the line, so the ThemeSel output pane will be like: (Theme acme, Theme a, Theme b) seperated by lines so the n/N will automatically consider selecting the Theme + argument (this is not output pane specific, it should be generalized to work on every pane kind), and will Exec it. Add new themes that will be automatically exported from helix and zed, a new binary will execute that will generate .zig files, this binary will be added as a execute step on the build.zig so when building it will regenarate those .zig that will be then included on the runtime as themes.