CacheIndexer observes PlayerController.uiState and records each played
track into audio_cache_index (source=INCIDENTAL), bumping lastPlayedAt
on re-play. This gives the offline ShuffleSource pools their data
source — previously the index was never written, so the pools were
inert. MediaItems now set a custom cache key of trackId so Media3's
SimpleCache is keyed per track (sets up slice 2's residency queries).
Intentional divergence from Flutter's file-per-track index: SimpleCache
(span-based) is the real byte store, so the index is a lightweight
play-recency record. Slice 2 (#33) queries SimpleCache for true
residency + size + eviction-sync to drive the cached indicator dot.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
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