Refactor queueNeedsBuild Bool into explicit transport sync state machine

[joshhbk]

Problem

queueNeedsBuild currently encodes multiple transport-sync meanings behind a single Boolean:

• pending sync at natural boundary
• pending sync on next play/resume
• sync in progress / retry in progress
• degraded or recovered but not fully disarmed

That ambiguity is making boundary-swap behavior brittle and has already produced subtle state bugs.

Current failure modes

1. pendingSkip can re-arm boundary swap even after queueNeedsBuild was cleared, allowing a later hard pause when no sync is needed.
2. Deferred sync can be stranded after in-app pause + external resume because re-arming is tied to add-time playing state rather than explicit sync intent.
3. Polling/arming/disarming logic is distributed across many call sites, which makes transitions hard to reason about and test.

Proposal

Replace queueNeedsBuild: Bool with an explicit transport sync state enum in domain state.

Example shape:

enum TransportSyncState: Equatable, Sendable {
    case inSync
    case pendingOnPlay
    case pendingAtBoundary
    case syncingAtBoundary
    case retryPending(strategy: SyncStrategy, attempts: Int)
}

enum SyncStrategy: Equatable, Sendable {
    case onPlay
    case atBoundary
}

Behavior goals

• Add while playing => .pendingAtBoundary
• Add while paused/stopped/no queue => .pendingOnPlay
• Boundary poller runs only in .pendingAtBoundary + .playing
• Skip/pause/remove/algorithm-change transitions become explicit and local
• Retry success/failure transitions cannot leave stale armed states

Incremental migration plan

1. Add transportSyncState to QueueEngineState with compatibility shim:
   • keep queueNeedsBuild as computed bridge during migration
2. Migrate reducer intents (addSong, addSongsWithRebuild, play, pause, syncDeferredTransport, resyncActiveAddTransport)
3. Migrate ShufflePlayer boundary-swap orchestration to consume enum states
4. Remove Boolean once all call sites and tests are migrated
5. Add transition tests:
   • paused add does not boundary-arm
   • external resume re-arms when pending-on-play
   • retry success clears armed/pendingSkip paths

Acceptance criteria

• No path can trigger boundary pause/swap when sync state is inSync
• No deferred sync intent can be stranded across pause/resume or external playback controls
• Boundary polling is only active when required by state
• Existing queue/transport invariant tests remain green; new transition tests added