Harden DoP playback transitions

[markojurkovic]

Summary

This PR hardens DoP playback transitions and Core Audio output preparation.

• Stops playback cleanly if the output device cannot prepare the next track’s format.
• Clears stale DoP seek, marker, and fader state when transitioning back to PCM.
• Enables DoP fader mode only after output preparation succeeds.
• Preallocates integer-output scratch space outside the real-time render callback, avoiding memory allocation during rendering.
• Restores the output reset state when format preparation fails.

Motivation

Failed output preparation or a DoP-to-PCM transition could leave stale DoP state active. This could cause PCM data to be handled as a DoP carrier, replace output with DoP silence, or leave playback in an inconsistent state.

The integer-output path could also resize its scratch buffer inside the Core Audio render callback, which is unsafe for real-time audio processing.

Test plan

• Build Cog in the Debug configuration.
• Test transitions from DoP playback to PCM.
• Test consecutive DoP tracks and DoP seeking.
• Verify playback stops cleanly when output format preparation fails.
• Verify integer DoP output continues without render-buffer allocation.