fix(subtitles): preserve secondary track identity across rebuilds

[orut34iop]

Summary

The audio-switch rebuild restores a primary registered subtitle through its registry id, but restores its secondary companion through only the URL. This loses the external identity, explicit headers and selected stream inside a subtitle container. Restore the secondary companion through its existing registry entry too, with coherent teardown and load ownership.

What changed

• Snapshot the secondary external id before internal teardown and re-select that id after rebuilding. One-shot sidecars retain the URL fallback, embedded tracks retain their source-time anchor.
• Clear a stale sidecar URL when selecting embedded subtitles, and clear active external identity on internal teardown while preserving the rebuild registry.
• Guard late secondary decode success and failure publication with the captured load generation, in addition to task cancellation.
• No new public API, bitmap/ASS capability, prepare/commit transaction, or restriction on using one track in both channels.

Test plan

Host: Apple Silicon Mac17,3, macOS 26.7.1, Swift 6.4. Existing synthetic multi-subtitle container fixture; tests verify the selected Spanish stream and registry identity after restoration, teardown, removal, stale-generation publication, embedded selection and same-track dual-channel behavior. Tests exercise the restoration routine called by the audio rebuild, not a physical audio-track switch or an AVPlayer media session. No new device acceptance is claimed for this isolated PR.

• Regression proof on untouched upstream production code: three tests failed with four assertions covering retained external id, stale sidecar URL, and stale decode publication into the next load generation.
• With the fix, 30 focused Swift Testing regressions across six suites passed under LIBDISPATCH_COOPERATIVE_POOL_STRICT=1, including multi-stream restoration and same-track dual-channel compatibility.
• Full default-traits run passed: 795 XCTest cases (one existing skip), 4,619 Swift Testing tests in 633 suites, zero failures.
• Native host build passed. Test bundle and CLI were signed and verified with an Apple Development identity; linker ad-hoc signing was disabled.
• Documentation links and git diff --check passed. Platform builds and all-traits coverage remain for upstream CI.

Checklist

• CHANGELOG and API behavior documentation updated
• Conventional Commit
• Engine fix, reuses existing registry
• No new public API or dependencies