0.20.0

AudioPlaybackKit 0.20.0 reads position on demand instead of pushing it every 100 ms, so a playing transport
wakes only for work and for the observers an app registers (#189,
ADR-0031).

This is a minor bump at 0.x: it changes public API (below). Decode and the PCM played are unchanged.

What changed

• Position is computed when read, as media3 Player.getCurrentPosition and AVPlayer currentTime().
  Transport.position (a TransportPosition: item, seconds) maps the output clock through a snapshot
  under the transport's lock, on the caller's thread with no hop to its queue. It is nil while a
  setCurrent or seek is called but not taken, and within one item and one call it never goes back.
  Player.position reads it once every call is answered and the heard item is the current one; until then
  it holds the value set with the last discontinuityCount change.
• Observers, named as AVPlayer's. addPeriodicObserver(interval:using:) fires every interval of
  output time while the clock runs (wall time at any speed). addBoundaryObserver(times:using:) fires once
  when playback crosses a time in the heard item; a load or seek landing past it does not. Both are on
  Transport (callbacks on its queue) and Player (callbacks on the main actor), and return a
  TimeObserverToken that removes the observer on cancel() or deinit.
• Events. TransportEvent.landed(item:seconds:) is the first position of a setCurrent or seek,
  once its first audio is written; seekLanded and seekAdjusted come from it.
  TransportEvent.durationChanged(item:duration:isSeekable:) is sent when the heard item's duration or
  seekability changes. firstAudioRendered is sent on .status(.playing) after a landing, or on a landing
  or .crossed while playing.
• NowPlayingSession still writes elapsed time only on a jump, a play or pause, a rate or item change,
  or drift. After a write while playing it asks the player for one periodic tick, so a play heard late is
  rewritten once heard; a paused or held player wakes nothing.
• Test support. ManualPumpTimer.advance moves the clock in 1/64 s steps, so an output's ready wake
  lands as it would on a real clock.

Breaking changes

• TransportEvent.position(item:seconds:duration:isSeekable:) is removed, replaced by .landed and
  .durationChanged: code matching it, or an exhaustive switch, no longer compiles.
• Player.position is computed, so SwiftUI and withObservationTracking no longer see it move between
  discontinuities. A view that showed it by observation stops updating.
• NowPlayingControllable requires addPeriodicObserver(interval:using:). A
  ForwardingNowPlayingControllable wrapper gets it forwarded; a direct conformer must forward it to its player.

Additive: Transport.position, TransportPosition, TimeObserverToken, and both observers on Transport
and Player.

Upgrading

• A progress bar or elapsed label subscribes with player.addPeriodicObserver(interval:using:), keeps the
  token, and reads player.position on each fire.
• Ad and silence skips and a sleep timer at an item time use addBoundaryObserver(times:using:) in place of
  comparing each .position.
• Code that read duration or seekability from .position reads .durationChanged, or Player.duration
  and isSeekable.