Darwin mediakeys

[bjarneo]

Extract a platform-agnostic MediaController interface from the MPRIS
integration so that media key support can be added per-platform without
modifying the core TUI code.

• mediactl/ package: Controller interface (Update, EmitSeeked, Close),
  MainLoopRunner interface for controllers that need the main OS thread,
  and a self-registration registry via init()
• MPRIS refactored to implement mediactl.Controller, self-registers via
  init() with blank import in main.go
• TrackInfo and SetPositionMsg moved to shared packages (mediactl and
  internal/control) so platform code doesn't cross-import
• Model uses []mediactl.Controller instead of *mpris.Service, with
  notifyMediaControllers() and emitSeeked() helpers replacing direct
  mpris calls
• prog.Run() moved to a goroutine with mediactl.RunMainLoop() on the
  main thread, enabling macOS NSRunLoop (no-op on Linux)
• Full macOS CGo scaffold: MPRemoteCommandCenter for receiving media
  key events, MPNowPlayingInfoCenter for Now Playing info, with change
  detection to skip redundant CGo calls on the per-second tick
• Key handlers upgraded from notifyMPRIS() to notifyAll() so Lua
  plugins also receive state updates on keyboard actions
• Notification hot path uses cached position/duration instead of
  re-acquiring the speaker lock

New:
Lock the main goroutine to OS thread 1 in init() and defer
MediaKeysInit to RunMainLoop so MPRemoteCommandCenter handlers
are registered on the correct thread for CFRunLoop delivery.