Extract shared core module, generate browser script at build time

DRY refactor:
- detect-antipatterns-core.mjs (297 lines): shared constants (SAFE_TAGS,
  OVERUSED_FONTS, GENERIC_FONTS, ANTIPATTERNS), color utilities (parseRgb,
  relativeLuminance, contrastRatio, hasChroma, getHue, colorToHex,
  isNeutralColor), and pure detection functions (checkBorders, checkColors,
  isCardLikeFromProps).
- CLI (889 lines, was 1212): imports from core, keeps jsdom-specific
  resolveBackground, page-level analyzers, regex fallback, and CLI logic.
- Browser wrapper (335 lines): template with browser-specific DOM adapters,
  highlighting, scan loop. Core is injected at build time.
- build-browser-detector.js: reads core, strips exports, injects into
  wrapper, writes to public/js/detect-antipatterns-browser.js (generated).
- Build step added to scripts/build.js (runs before Bun bundling).

Source of truth for detection logic is now the core module. Browser script
is generated — do not edit public/js/detect-antipatterns-browser.js directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

scripts/build.js

@@ -322,6 +322,13 @@ async function build() {
   // Build CSS with Tailwind CLI (handles @theme directive)
   buildTailwindCSS();
 
+  // Generate browser anti-pattern detector from core module
+  try {
+    execSync('node scripts/build-browser-detector.js', { cwd: ROOT_DIR, stdio: 'inherit' });
+  } catch (error) {
+    console.error('Failed to build browser detector:', error.message);
+  }
+
   // Bundle HTML, JS, and compiled CSS with Bun
   await buildStaticSite();