Davy7/20/2026, 3:36:38 PM

Add an async stale-guarded stem-set download orchestrator, a testable
playback reconnect coordinator, and an atomic source-swap command so a
remote streaming source can recover from transient mid-song failures
without an audible position jump.

- errors: add RemoteErrorKind::StaleRequest (never retried) for the
  stale-guard abort path.
- playback_source: add ensure_remote_stem_set_cached_guarded which checks
  a Fn() -> bool stale guard before each stem download and before the
  atomic-rename phase, discarding temps and returning StaleRequest when
  the active request has moved on. The playback path now uses the guarded
  variant.
- reconnect: new services::reconnect module with a bounded-retry
  ReconnectCoordinator driven by injected closures (re-resolve, seek,
  credential refresh, stale guard, event sink, sleep). Reuses the shared
  net_policy full-jitter backoff. Emits Reconnecting/Resync/Failed events;
  classifies via RemoteErrorKind so non-transient errors abort immediately
  and credential-expired errors trigger a refresh before the next attempt.
- coordinator: add ReplaceStreamingSource PlaybackCommand and handler that
  swaps the active streaming source under the playback mutex in one
  critical section, sets render_frame to the preserved position, seeks the
  new consumers, and marks buffering. Stale requests (user skipped) no-op.
- playback: wire the reconnect coordinator into the fetch event thread.
  ConsecutiveFailures triggers a transient reconnect; UrlExpired triggers
  a credential-expired reconnect; RangeNotSupported keeps the full-file
  fallback. On success the new source is sent to the coordinator for
  atomic swap; on failure a remote-playback-failed event and a
  playback-error are surfaced.
- playback: add remote-playback-reconnect / remote-playback-resync /
  remote-playback-failed IPC events and payload structs.
- docs: document the three new events and the internal
  ReplaceStreamingSource command in the playback contract.

Tests: 8 reconnect unit tests (transient retry, budget exhaustion,
non-transient abort, credential refresh, resync, stale guard, cache fast
path, classification) and 3 stale-guard stem-set tests (rename abort,
mid-set cancellation, current-request success).

Parent87b2055