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/candidate-b.css

@@ -0,0 +1,15 @@
+/* B — Two Metals. A + pewter #9EA7B3: pewter structures, gilt marks what matters or can be touched. */
+:root {
+  --wg-pewter: #9EA7B3;
+  --wg-pewter-rgb: 158, 167, 179;
+  /* structure moves to pewter */
+  --wg-border: rgba(var(--wg-pewter-rgb), 0.20);
+  --wg-border-soft: rgba(var(--wg-pewter-rgb), 0.11);
+  --wg-ledger-ruling: rgba(var(--wg-pewter-rgb), 0.075);
+  --wg-ledger-rule-strong: rgba(var(--wg-pewter-rgb), 0.20);
+  --wg-ledger-border: rgba(var(--wg-pewter-rgb), 0.24);
+  /* metadata is structure, not value */
+  --wg-text-muted: #9EA7B3;
+  --wg-ledger-ink-muted: #9EA7B3;
+  /* gilt keeps: links, ledger ink, buttons, focus ring, stamps */
+}