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, 6 insertions, 0 deletions

diff --git a/src/config.zig b/src/config.zig
index c4249270..35be4b9f 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -108,6 +108,11 @@ pub const leader_path = std.EnumArray(Builtin, ?[]const u8).init(.{
     .Colors = "tc",
     .NextColor = "tn",
     .Crt = "tr",
+    // the theme picker joins the toggles it belongs with; `Theme` itself takes
+    // a NAME, and a key path can never carry one, so it has none (the same
+    // reason Look and Exec have none)
+    .ThemeSel = "tt",
+    .Theme = null,
     // the image toggles join the same `t` group; Palette takes `l` because
     // `p` is Petscii's and `c` is Colors'.
     .Petscii = "tp",
@@ -401,6 +406,7 @@ pub const symbol_marker = " WsSymbols /";
 pub const search_buffer = "+Search";
 pub const help_buffer = "+Help";
 pub const jumps_buffer = "+Jumps";
+pub const themes_buffer = "+Themes";
 pub const hover_buffer = "+Hover";
 pub const lsp_buffer = "+Lsp";