feat(theme-selector): add browser-based theme design tool

A self-contained tool for building Emacs color themes by eye. generate.py emits one HTML page with six languages of tree-sitter-tokenized code, a category-to-color assignment table, a UI-faces table, and an editable palette. Reassign colors from the palette, toggle weight and slant per category, set foreground and background per UI face, then export a theme.json a later build step turns into theme files.

The export carries the name, palette, syntax assignments, bold and italic sets, and a ui object of per-face foreground and background. The theme name is both the json name field and the download filename.

samples.py holds the language samples and the default color map. theme-selector.html is the generated output. The json-to-theme converter is the next piece, and the part worth TDD.

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+# theme-selector
+
+A self-contained tool for designing Emacs color themes by eye. It renders six
+languages of tree-sitter-tokenized code, a category→color assignment table, a
+UI-faces table, and an editable palette into one HTML page you drive in the
+browser. Reassign colors, toggle weight/slant, edit the palette, then export a
+`theme.json` that a build step turns into `themes/<name>-*.el`.
+
+## Run
+
+```bash
+python3 generate.py          # writes theme-selector.html beside this script
+```
+
+Then open it in Chrome (Firefox had color-rendering flakiness during design):
+
+```bash
+WAYLAND_DISPLAY=wayland-1 google-chrome-stable theme-selector.html
+```
+
+During color work, disable Hyprland inactive-window dimming so colors read true:
+
+```bash
+hyprctl keyword decoration:dim_inactive false
+```
+
+## Files
+
+- `generate.py` — emits the HTML+JS. Edit here to change layout or behavior.
+- `samples.py` — the language code samples and the default category→color map
+  (`COLS`). `generate.py` reads the part before the `cols=` marker.
+- `theme-selector.html` — generated output. Regenerate; don't hand-edit.
+
+## What it captures
+
+- Background and foreground (the `default` face's `:background` / `:foreground`).
+- The syntax layer: every font-lock / tree-sitter category (keyword, string,
+  function, type, comment, and the rest), each with normal/bold/italic.
+- UI faces: cursor, region, mode-line, fringe, line numbers, isearch, paren
+  match, link, error/warning/success, and more — foreground and background per
+  face.
+- The palette itself: add by hex or swatch, remove, rename, drag to reorder.
+
+## theme.json contract
+
+The export (and what a build step consumes):
+
+```json
+{
+  "name": "dupre-revision",
+  "palette": [["#67809c", "blue"], ["#e8bd30", "gold"]],
+  "assignments": {"kw": "#67809c", "str": "#5d9b86", "bg": "#0d0b0a", "p": "#cdced1"},
+  "bold": ["kw", "fnd"],
+  "italic": [],
+  "ui": {"region": {"fg": null, "bg": "#264364"}, "cursor": {"fg": null, "bg": "#a9b2bb"}}
+}
+```
+
+The theme name is both the `name` field and the download filename
+(`<name>.json`, sanitized). Upload a `theme.json` to start from a prior theme.
+
+## Next step (not yet built)
+
+A `theme.json` → `themes/<name>-palette.el` + `-faces.el` + `-theme.el`
+converter. That step is the correctness-sensitive part and is the one worth
+TDD: JSON in, valid Emacs palette + faces out, every face mapped, WCAG-contrast
+asserted on the result.