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/probe.js

@@ -0,0 +1,48 @@
+// Did the candidate CSS actually reach the pixels? Computed-style probe for #53.
+const fs = require('fs');
+const path = require('path');
+const { chromium } = require('playwright');
+const BASE = process.env.BASE || 'http://localhost:4567';
+const read = (f) => fs.readFileSync(path.join(__dirname, f), 'utf8');
+const base = read('_base-plumnoir.css');
+const CANDIDATES = { baseline: '', a: base, b: base + read('candidate-b.css') };
+
+const PROBES = [
+  ['body', 'backgroundColor'],
+  ['[component="post"]', 'backgroundColor'],
+  ['[component="post"] .post-header, [component="post"] .topic-item-header', 'color'],
+  ['.timeago', 'color'],
+  ['[component="post"]', 'borderTopColor'],
+  ['.topic-title, h1', 'color'],
+  ['a[href^="/user/"]', 'color'],
+];
+
+(async () => {
+  const browser = await chromium.launch();
+  const page = await browser.newPage({ viewport: { width: 1600, height: 1000 } });
+  await page.goto(BASE + '/login');
+  await page.fill('#username', 'admin');
+  await page.fill('#password', 'Admin12345!');
+  await page.click('#login');
+  await page.waitForLoadState('networkidle');
+
+  const rows = {};
+  for (const [name, css] of Object.entries(CANDIDATES)) {
+    await page.goto(BASE + '/topic/2', { waitUntil: 'networkidle' });
+    if (css) await page.addStyleTag({ content: css });
+    rows[name] = await page.evaluate(
+      (probes) =>
+        probes.map(([sel, prop]) => {
+          const el = document.querySelector(sel);
+          return el ? getComputedStyle(el)[prop] : 'MISSING';
+        }),
+      PROBES,
+    );
+  }
+  PROBES.forEach(([sel, prop], i) => {
+    console.log(
+      `${sel} ${prop}\n  baseline ${rows.baseline[i]}\n  a        ${rows.a[i]}\n  b        ${rows.b[i]}`,
+    );
+  });
+  await browser.close();
+})();