themes: one file each, generated from helix and zed, picked by stepping a list

Each theme is a .zig file of pure data and nothing enumerates them by hand —
fold() walks the container's declarations, so a theme is a file and that is the
whole registration. Field by field rather than a wholesale coercion, which
makes a missing field a compile error that names it.

tools/gen_themes.zig reads helix .toml and zed .json out of vendor/themes and
emits one .zig each; build.zig reads that directory, so adding a theme is
dropping a file in. Output goes to the build cache rather than the tree, so zig
owns the freshness check and a deleted source cannot leave a stale theme
behind. Vendored, not read from the genizah: the build stays offline. A source
it cannot map is a hard error naming the file and the key, never a silently
black-on-black theme.

ThemeSel is the clever half. Traits gained an `executes` column, so n/N over
that buffer hands the whole line to Exec instead of the leading word to Look —
both arms the ordinary builtin, so a stepped row does exactly what the matching
mouse button on it would. Stepping the list previews each theme live. The trait
is a pane property, not an output-pane branch, so any pane whose lines are
commands can opt in.

Goldens: leader gains SPC t t; theme's fourth NextColor no longer wraps to
helix because the ring is fifteen long. New: themesel.

1 files changed, 13 insertions, 7 deletions

diff --git a/docs/design.typ b/docs/design.typ
index 7a9cc5c7..5d6daad2 100644
--- a/docs/design.typ
+++ b/docs/design.typ
@@ -243,13 +243,19 @@ visible-range first), Save, open-at-line, undo/redo. Image: zstbi decode,
 kitty graphics when available, petscii matcher fallback (C64/terminal
 palettes, ascii glyph set toggle). Tutor: embedded text as file pane.
 
-*Chrome.* Themes helix (default; near-black page, chrome one grey-ramp step
-off it, colors lifted from the helix editor's own theme), dark, acme-light —
-the first two leave a child's ANSI palette native, acme-light resolves it onto
-the page so shell output stays readable on yellow. A theme owns the page, the
-tag bar, the gutter and the move box; the selection colors stay fixed because
-they encode which button you pressed. Colors toggles all recolor passes; Debug
-stats overlay; Dump writes state ZON.
+*Chrome.* One theme per `.zig` file, folded into the ring at comptime: ours
+first — helix (default; near-black page, chrome one grey-ramp step off it,
+colors lifted from the helix editor's own theme), dark, acme-light — then
+everything `tools/gen_themes.zig` exports at build time out of the vendored
+helix `.toml` and zed `.json` sources in `vendor/themes`. helix and dark leave
+a child's ANSI palette native; acme-light and the zed exports resolve it onto
+the page so shell output stays readable on a light one. A theme owns the page,
+the tag bar, the gutter and the move box; the selection colors stay fixed
+because they encode which button you pressed. `NextColor` walks the ring,
+`Theme <name>` jumps to one and `ThemeSel` (`SPC t t`) lists them all into an
+output buffer whose rows are those very commands — so n/N EXECUTE each row
+instead of looking it and stepping the list wears the themes. Colors toggles
+all recolor passes; Debug stats overlay; Dump writes state ZON.
 
 *Web (replay viewer parity).* Embedded dump; focus, border/move drags, wheel,
 scrollbar, Colors/NextColor, pinch, render scale, and link-LOOK opening a new