Optimize subtitle HLS renditions for AirPlay

[hikaps]

Summary

• eliminate duplicate video output when multiple subtitle renditions are present
• generate stable WebVTT HLS subtitle playlists and deterministic selected-subtitle markers
• keep AirPlay route-sharing policy and native subtitle selection behavior consistent

Verification

• ./gradlew :composeApp:jvmTest :composeApp:koverVerify --console=plain
• ./gradlew :composeApp:iosSimulatorArm64Test --console=plain
• complete unsigned Swift XCTest suite on iPhone 16 Pro iOS 18.2 simulator

Physical AirPlay receiver validation remains to be performed.

[hikaps]

Addressed review findings in commit bdc6950:

• omit SUBTITLES="subs" when no subtitle renditions exist
• align WebVTT output to the media timeline with empty gap segments and bounded target duration
• publish empty valid subtitle playlists before the first cue
• preserve valid sidecars that are empty after a seek; fail only decode errors
• retain all renditions for decoder cleanup

Verification passed:

• complete Swift XCTest suite on iPhone 16 Pro iOS 18.2 simulator
• JVM tests + Kover
• Kotlin iOS simulator tests
• review-specific late-cue and no-subtitle-master tests

[hikaps]

Resolved the remaining timeline findings in commit f42e4d5:

• subtitle renditions now use fixed four-second video-timeline periods
• intersecting cues retain their original timestamps across every affected period
• each WebVTT segment includes X-TIMESTAMP-MAP aligned to the A/V timestamp origin
• overlapping cues are retained instead of advancing from the previous cue end
• #EXT-X-TARGETDURATION:4 is fixed from initial publication through finalization

Added regression coverage for late cues, gap periods, original timestamps, timestamp mapping, and fixed target duration. Full Swift XCTest, JVM/Kover, and Kotlin iOS simulator suites pass.