feat(session): resolve the audio track from an ordered language preference

[bilipp]

Summary

Engine half of a paired change for bilipp/Lume#200 — a preferred-audio-language setting in the Lume app. This adds the API the app drives and resolves the audio track against it at open time.

Companion app PR: bilipp/Lume#211.

Implementation

PlayerConfiguration gains two fields, both defaulting to today's behaviour:

• preferredAudioLanguages: [String] — ordered, already-normalised bare tags supplied by the caller. The engine does not normalise; the app does, and passes ["de", "en"].
• autoEnableForcedSubtitlesForForeignAudio: Bool — off by default.

buildPipeline(info:) resolves the audio track against that preference before demuxer.resume(), falling back to the existing first(where: \.isDefault) ?? first when the list is empty or nothing matches. With an empty list the resolution is byte-for-byte what it was.

Why at open, not after

Selecting after open() routes through selectAudioTrack → seek(to: position), and position is 0 until the first frame — which silently wipes a configuration.startPosition VOD resume. Worse for live: MediaInfo.isSeekable is true whenever pb->seekable != 0, which many live IPTV HTTP endpoints report, and providers drop the connection on the seek. Resolving during buildPipeline costs no lane teardown and no seek.

Matching

TrackLanguageMatcher carries the ISO 639-2/B alias table Foundation does not provide — Locale.LanguageCode("ger").identifier(.alpha2) is nil while "deu" resolves, and ger/fre/chi/cze/dut/gre are exactly what IPTV containers ship. Candidates are scored by preference rank, a commentary / audio-description penalty, and exact-over-regional-variant; track titles are matched per whitespace/punctuation token, never by substring, so GER 5.1 matches but Hungarian does not match a hu-seeking pass via gar. und / mul / mis / zxx / vo / vost / multi / original resolve to nothing, which means "leave the container's choice alone".

Testing

• swift build clean
• swift test — 85/85 tests in 14 suites pass
• Tests added — TrackLanguageMatcherTests plus four PlayerSessionTests cases asserting selectedAudioTrackIndex after open() with a preference set
• Empty-preference path asserted to keep the previous default selection

Tests/…/Fixtures/generate-fixtures.sh now generates the multi-audio-language MKV PLAN.md's testing section already anticipated (two audio streams tagged eng / ger, eng dispositioned default). Generated media stays uncommitted, as with the other fixtures.

Reviewer notes

• Additive public API only — two public vars with defaults on an existing public struct that has a public init(), safe under the package's -enable-library-evolution.
• Must land with feat(player): auto-select the viewer's preferred audio language Lume#211. The app resolves ../LumeEngine as a local SPM package, so that branch does not build against an engine checkout without this commit. Sideload CI needs an ENGINE_REF bump once this is tagged — deliberately not done here, since pointing the workflow at a tag that does not exist yet breaks the build harder than the current skew.
• A near-twin of TrackLanguageMatcher lives app-side. That duplication is deliberate for now: sharing it would mean a new SPM target plus a package-product dependency on the app and test targets. Worth revisiting if the two drift.