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.

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