The Episode Focus View's episode strip collapsed to just the current
episode on some series. Two causes:
- Series that expose episodes directly as children rather than under
season folders yielded an empty season fetch, leaving allEpisodes
empty. The library page now groups those flat episode children by
their season number and synthesizes minimal season headers.
- isCurrentEpisode over-matched: episodes with no season/episode number
compared equal (undefined === undefined) and every one of them looked
like the focused episode.
Extracts the strip's pure logic into episodeStrip.ts so both behaviours
are unit-tested, per the failing-test-first rule.
TRACES: UR-058 | DR-087
