Groundwork for #1300. Establishes the session, buffering and route
handling Dolby's application guide prescribes, and adds the diagnostic
arm needed to find out whether Apple's sample-buffer renderer can carry
Atmos objects at all.
Audio session, per the guide's sequence:
- Adopt the long-form playback profile in one atomic call at app launch
and activate the session there. The SDK only accepts that policy with
category Playback, a Default/MoviePlayback/SpokenAudio mode and no
options, so it cannot be assembled from separate calls.
- Report the resolved rendering mode in the player, hidden unless the
system resolves it. Apple only resolves it for CarPlay and AirPlay, so
an unresolved value means unknown, never "not Dolby".
Diagnostics (Apple TV only, Settings > Video Playback > Atmos Output Test):
- Add a sample-buffer arm. It reads the asset with AVAssetReader at
outputSettings nil and hands the untouched compressed buffers and the
untouched format description straight to the renderer, with a variant
that rebuilds the description the way playback builds it. Every
existing mode went through AVPlayer, so nothing exercised the path
playback actually uses; this is what tells us whether the renderer or
our construction is at fault.
- Add an AirPlay route picker. AirPlay is the only route where the system
resolves the rendering mode and the supported channel layouts, so it is
what makes those observations reachable at all, and the AVPlayer arms
now allow external playback so every arm can be compared on the same
destination.
- Add a session-mode toggle for the one profile difference between the
guide and previous playback behaviour.
- Report the session profile, supported layouts, both format
descriptions, the magic cookie and the renderer status, and release the
session on stop so a failed run cannot contaminate the next one.
Also bumps MPVKit to 1.0.14, which carries the matching audio output
work: the channel layout AVFoundation itself uses for Dolby content, a
renderer-failure observer so the fallback to PCM can actually run, the
prescribed feed ordering and preroll, flush recovery that re-supplies the
discarded audio instead of shifting later audio into its place, and
capability-driven fallback on route and capability changes.
This does not yet fix#1300. Whether the sample-buffer renderer can carry
JOC is still unknown; it removes every difference from the documented
setup that could explain the failure, and gives us the arm to answer it
on real hardware.
edde746