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>

.gitignore

@@ -45,6 +45,11 @@ extension/shared/
 website/www/
 website/.avif-cache.json
 
+# Playwright E2E artifacts (the fixtures themselves are committed)
+test-results/
+playwright-report/
+blob-report/
+
 # Temporary scratch files
 scratch/