fix(subtitles): preserve secondary sidecar ASS metadata

[orut34iop]

Summary

preserveASSMarkup already preserves primary sidecar ASS events and their script header, but secondary sidecars silently use the default text decoder and discard the header. Honor the same opt-in option on the secondary channel and publish secondarySidecarASSHeader independently.

What changed

• Secondary decoding passes the existing option to SubtitleDecoder; the existing ASS/SSA codec gate keeps SRT/VTT output unchanged.
• Reset the secondary header on a new sidecar, embedded selection, clear and stop. Publication stays behind the existing cancellation/active-channel checks.
• Document which header belongs to which cue list. No renderer, forced default styling, channel-exclusivity rule or new embedded decoding path.

Builds on the existing primary API from #48 and per-stream ASS decoding from #267. All changes are based on main 151322d1b843b4414a0fcffa9ed44e8b3a9d087e; #824 and #825 touch adjacent secondary state/comments but are separate contributions, so merge order may require textual reconciliation.

Test plan

• Host: Mac17,3, macOS 26.7.1 (25G241), Swift 6.4 / Xcode toolchain; default traits, native SwiftPM.
• Media: existing generated Matroska fixture with English ASS at 640×480 and Spanish ASS at 1920×1080, identical raw placement tags; generated SubRip sidecar. No private media or device playback claim.
• Regression coverage: two independent headers/raw events, default headerless output, ASS-to-SubRip/embedded selection, independent clear, cancelled publication, stop, carryover restoration and WebVTT staying text.
• Strict cooperative-pool focused run: 34 Swift Testing tests passed. Full serial run passed: 795 XCTest (one existing skip) and 4620 Swift Testing tests / 634 suites.
• The initial full parallel run failed only in the existing SoftwarePacketMemoryFIFOTests history-walk assertion with invalidCursor; reclaimable-history test assumptions are also addressed by open test(cache): respect kernel purges in history eviction checks #819. No unrelated tests were edited; failed evidence was retained.
• Documentation links and git diff --check passed.
• Valid Apple Development signing and verification for local test executables; no ad-hoc signing.

Checklist

• CHANGELOG.md updated
• Conventional commit
• Fix lives in the engine
• Public API documented