feat: rank player candidates by ordered signals, add browser E2E suite

The weighted score summed incomparable units, so size could outvote traits
that disqualify an element outright. Measured on a real page: a display:none
preload reports its full 1080p intrinsic size and scored 2073600, beating a
visible unmuted player at 509920.

Selection now compares an ordered list of signals, highest priority first:
has a source, is rendered, is not a silent background loop, rendered size
bucket, is playing, has controls, duration. Rendered size replaces intrinsic
resolution, and mute state is gone from the ranking entirely: it is a viewer
preference, not evidence about which element is the player.

It stays a ranking rather than a filter, so a page of only bad candidates
still yields one and findVideo never returns null where a video exists.

The new tests/e2e suite runs the shipped finder against real fixture pages
and drives the packed extension for injection, reinjection and remote
play/pause/seek into a first-party frame. All five scoring scenarios fail
against the previous implementation.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -46,3 +46,32 @@ jobs:
 
       - name: Run verification suite
         run: npm run verify
+
+  e2e:
+    # Kept separate from `verify`: this job needs a downloaded browser, so a
+    # failure here should read as "the browser flow broke", not as a broken
+    # lint or unit run. `verify` stays usable offline and in the release job.
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v7
+
+      - name: Set up Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: '24'
+          cache: 'npm'
+          cache-dependency-path: package-lock.json
+
+      - name: Install root dependencies
+        run: npm ci
+
+      - name: Install Playwright Chromium
+        run: npx playwright install --with-deps chromium chromium-headless-shell
+
+      # The extension specs load dist/chrome, so the artifact has to exist.
+      - name: Build the extension
+        run: npm run build:extension
+
+      - name: Run extension E2E smoke tests
+        run: npm run test:e2e