feat(presenter): add subtitle delay controls

[Shinokawa]

Summary

Extracts subtitle-delay support from #53 into a focused change.

• adds mpv-style subtitle-delay APIs through Rust, C ABI, platform plugins, and Dart
• shifts subtitle overlay lookup time at render, clock refresh, overlay refresh, and capture paths
• retains adjacent cues in the delayed display domain

Draft blockers

• negative delay still needs engine-level lookahead/delivery support; the current overlay-only lookup cannot display a cue before its unshifted delivery time
• increasing a positive delay during playback cannot restore cues that were already retired

This PR stays draft until those scheduler/retention semantics are completed.

Validation

• cargo fmt --all -- --check
• cargo test -p erika -p erika_capi --lib — 393 + 26 tests passed
• flutter test — 37 tests passed

Split from #53 to keep subtitle scheduling separate from mute and audio-clock work.