Playback probes: await metadata track replacement before progress checks

[jlipworth]

Scope

Fixes #312. Metadata-owned audio/subtitle probes wait for a nonnil replacement AVPlayerItem before readiness and sustained progress. Soft AVFoundation/offline subtitle switches do not require replacement. Unexpected replacement during the hold still fails; production restart, consent and decoder behavior are unchanged.

Rebased only this PR's own commits onto main b0f6c7c6, after PR310/313 landed. The audit Markdown table is programmatically padded to equal column widths including its separator; all original cells are preserved. Historical baseline, 27-report ledger and 1,699-test count remain explicitly point-in-time, with a concise durable integration note. No HDR or infrastructure changes.

Current rebased-head validation

• Full hermetic PMSKit suite: 1,705 tests passed; hygiene, strict MkDocs, links and Mermaid passed.
• Eight focused hosted evidence tests and semantic iPhone fixture smoke passed on a fresh build.
• Ordinary Plex Link authentication; exact same-source H.264 metadata audio and VC-1 metadata subtitle transitions passed with 30-second post-replacement holds and reviewed decoded video. The first subtitle run's samples had no text; retained that run and verified visible subtitle text on the next same-source selection. All three current reports passed; listening accuracy is not established.
• Fresh iPhone, visionOS, tvOS and isolated Mac builds and visually reviewed fixture smokes passed. Owned simulators/staged apps cleaned up; source bindings, frames and logs retained privately.

Limits

This focused revalidation is not a rerun of the historical 27-report corpus audit. Whole-title stability, exhaustive track combinations, audible multichannel correctness, hardware playback and HDR/colorimetry remain open. No server configuration or library-wide encoding changes. Awaiting parent exact-head review/merge; do not merge automatically.