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, 12 insertions, 3 deletions

diff --git a/test/snapshots/theme.snap b/test/snapshots/theme.snap
index ca0b22ba..399b1950 100644
--- a/test/snapshots/theme.snap
+++ b/test/snapshots/theme.snap
@@ -1,9 +1,18 @@
 # The theme ring, proved by style and not by text. Boot is "helix" (index 0,
 # the default): near-black page, tag bar one grey-ramp step off it, grey move
 # box. NextColor (still in the topbar) then walks to "dark" — the OLD default,
-# which has to stay reachable — then to acme-light, then back around to helix.
+# which has to stay reachable — then to acme-light.
 # Colors, which left the bar for its leader path, toggles the recolor passes
 # off and on from SPC t c — the same builtin either way.
+#
+# The fourth click used to come back around to helix, because the ring was
+# three long. It is not any more: everything vendor/themes exports follows the
+# three pardes ships with (pardes.zig: `curated ++ generated`), so the click
+# lands on the FIRST GENERATED theme instead. That capture is named for what it
+# is rather than for which theme it happens to be, because the generated half
+# is sorted by name and a theme dropped into vendor/themes can take the slot.
+# The wrap itself is `% themes.len` and clicking twenty times to see it is not
+# worth a golden; themesel.snap covers reaching a theme by name.
 file t.txt x
 start 30 100 -n 3
 wait 8000 t.txt
@@ -33,5 +42,5 @@ stable 700 10000
 press middle 25 1
 release middle 25 1
 stable 700 10000
-snap helix-again
-snapstyle helix-again
+snap generated
+snapstyle generated