Split PlayerSession into PlaybackEngine + three stores (fixes #60, #62)

[KyNorthstar]

What

• New PlaybackEngine — owns the AVPlayer, remote transport
  commands, Now Playing info, the periodic time observer, per-item
  end/time-jump sinks, restored-seek readiness KVO. isPlaying (and
  every other fact) is written in exactly one place.
• New NowPlayingStore, HistoryStore, PlaylistLibraryStore — the
  three domains PlayerSession is a god-object — three unrelated domains in one class #62 already named, each owning its own persistence.
• PlayerSession (779 → 336 lines) is now a thin facade composing
  all four.
• MediaPlayerView (634 → 238 lines) owns no playback state — reads
  and displays session.engine. Adds init(playing: URL) /
  init(playing: [URL]) for genuinely drop-in single-file use.

Why

Fixes #60 by construction: nothing outside PlaybackEngine writes
isPlaying. Fixes #62 by not adding a fourth responsibility to the
god object while fixing #60 — splitting now instead.

Concrete payoff beyond tidiness: requestPlaybackOnNextLoad() /
takePlaybackIntent() / takePendingRestoredSeek() are deleted.
Their own doc comments explained they existed only because "the
session doesn't own the player" — once NowPlayingStore and
PlaybackEngine are composed together, that's no longer true, so
there's nothing left to park.

Naming: a Store is durable data independent of any player instance
and persists itself; the Engine is live machinery that exists only
while something is loaded and persists nothing. PlaybackEngine, not
Player — that name's taken by the existing AVPlayerViewController
wrapper.

Behavior changes (intentional)

• Tapping the already-current queue entry now plays it if paused,
  rather than no-opping — the no-op only existed to avoid stranding a
  parked intent that no longer exists.
• Remote-control-event registration now follows the engine's
  play/unload rather than the view's onAppear/onDisappear —
  batteries no longer depend on a specific View staying alive.

Deliberately not done here

• No unit tests. This is what makes 0.0.7 #28 possible (PlaybackEngine
  with its delegate is trivially testable outside a View), but that's
  its own PR.
• MediaPlayerView still draws the title/creator block, so nightly
  doesn't regress before the pinned-header UI lands.
• The reactivity model mixes three paradigms, mostly by accretion rather than by decision #65 untouched — the Combine/KVO/@Observable mix moved into the
  engine unchanged.

Verification

No Xcode in my environment. Verified by grepping for every removed
symbol across the tree and tracing every call site by hand — not by
building. A real build is still the first thing this needs.

Journal: LLM transparency/Claude Fable 5.1 • Issues #60 & #62 journal.md

Fixes #60
Fixes #62
Depends on #77