DRY: replace Puppeteer page.evaluate with browser script injection, add browser tests

Major cleanup:
- detectUrl() now injects the browser script via page.evaluate() and
  calls window.impeccableScan() instead of reimplementing all detection
  logic inline. Removes ~80 lines of triple-duplicated code.
- Removed dead isPureBlackOrWhite function.
- CLI reduced from 1286 to 1212 lines.

New: Puppeteer-powered browser parity tests (detect-antipatterns-browser.test.js):
- Starts a local HTTP server for fixtures
- Loads fixture pages in headless Chrome
- Runs the browser detection script via impeccableScan()
- Verifies findings match expectations for all fixture categories:
  borders, colors, layout, typography, partials

8 new browser tests catch desync between CLI and browser script
(like the WeakSet iteration bugs we hit earlier).

puppeteer added as devDependency.

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

package.json

@@ -32,6 +32,7 @@
   "type": "module",
   "devDependencies": {
     "jsdom": "^29.0.0",
+    "puppeteer": "^24.39.1",
     "wrangler": "^4.71.0"
   }
 }