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/_base-plumnoir.css

@@ -0,0 +1,16 @@
+/* shared by all candidates: the Plum Noir surface lineage (#49 rule "re-tune, don't replace") */
+:root {
+  --wg-bg: #150E13;
+  --wg-bg-deep: #0C0810;
+  --wg-bg-deep-rgb: 12, 8, 16;
+  --wg-bg-deepest-rgb: 8, 6, 11;
+  --wg-panel: #21161C;
+  --wg-panel-2: #180F16;
+  --wg-panel-3: #351E28;
+  --wg-plum: #351E28;
+  --wg-plum-rgb: 53, 30, 40;
+  --wg-plum-soft: #4A2A38;
+  --wg-velvet-panel: rgba(33, 22, 28, 0.97);
+  --wg-velvet-panel-hover: rgba(45, 30, 38, 0.98);
+  --wg-ledger-panel: rgba(24, 15, 22, 0.97);
+}