playback: stop giving ExoPlayer the whole queue

Queue ownership moves from ExoPlayer into ExoPlaybackStateHolder. The
player now only ever holds a window of up to 51 MediaItems centered on
the current song, slid as playback advances; the full queue lives in
plain heap/mapping/index fields that resolveQueue() snapshots, in the
same representation RawQueue always used.

Handing the player a MediaItem per queue entry was the largest single
consumer behind the large-library OutOfMemoryErrors: measured at ~77MB
of Java heap for a 40k-song "play from all" queue, transiently doubled
on every reshuffle, on top of a library model that already runs ~100MB
at that size. Playback buffers allocate from the same heap, which is why
the reported traces died inside PlaylistTimeline construction and
shouldContinueLoading.

Measured on device with the library inflated to 40194 songs:

  steady heap after shuffle-all   118MB -> 53MB (baseline 47MB)
  transient peak                  159MB -> 83MB
  after skips + 4 reshuffles      219MB -> 89MB

Notes on the mechanics:
- Queues that fit entirely in the window behave exactly as before,
  including native repeat handling.
- On larger queues RepeatMode.ALL is emulated by letting the window wrap
  circularly around the queue edges, so skips and automatic advances
  cross the seam without the player ever repeating on its own.
- Window slides append/remove at the edges only, so gapless preloading
  of the next song survives ordinary playback. Queue mutations rebuild
  the window around the current item without interrupting it.
- BetterShuffleOrder and the timeline unscrambling walk existed only to
  read the queue back out of the player, and are deleted; the shuffle
  mapping is now generated and owned directly.

Verified on device: shuffle-all, 30 rapid skips across the window edge,
prev, and state persistence across process death all behave; the media
session queue index tracked exactly through all of it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmarca-tech