prototype(#53): palette candidates as runtime token swaps

Throwaway. Tokenises the theme's literal colours so candidate palettes can be
swapped in the browser, adds three candidates (Plum Noir one-metal, two-metal
pewter, two-metal plus a champagne inset), a WCAG checker, a seeder and the
screenshots they are judged on.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

prototype/README.md

@@ -0,0 +1,21 @@
+# Throwaway prototype for #53 — palette candidates on a dense page
+
+Not for merge. The theme SCSS on this branch is mechanically tokenised (literal
+hexes and `rgba()` triples turned into `var(--wg-*)`) so a candidate palette can
+be swapped at runtime instead of rebuilt per candidate.
+
+- `contrast.py` — WCAG check for every candidate surface/ink pair. Run first.
+- `_base-plumnoir.css` + `candidate-{a,b,c}.css` — the candidates, as `:root` overrides.
+- `seed.js` — categories + one 13-post topic with a quote, over the REST API.
+- `shots.js` / `probe.js` — screenshots and a computed-style probe, both against the live stack.
+- `shots/` — the images the resolution is judged on.
+
+Replay:
+
+```bash
+cd ../sow-nodebb && nix develop --command make dev-reset && nix develop --command make dev
+cd - && node prototype/seed.js
+export PLAYWRIGHT_BROWSERS_PATH=$(nix build --print-out-paths --no-link nixpkgs#playwright-driver.browsers)
+P=$(nix build --print-out-paths --no-link nixpkgs#playwright-test)
+NODE_PATH=$P/lib/node_modules nix shell nixpkgs#playwright-test -c node prototype/shots.js
+```