feat: add per-sound boundary effects

[han4wluc]

Adds reusable update effects at individual sound iteration boundaries.

The engine now:

• accepts flat beginEffect and endEffect selections on BGM, SFX, and voice sounds
• requires those selections to reference type: update audio-effect resources
• compiles volume, pan, and playback-rate keyframes directly onto rendered sound nodes
• applies selection playback speed while compiling keyframe timing
• leaves channel-level bgm.audioEffects transitions unchanged
• removes the discarded per-sound incomingTransition / outgoingTransition handoff contract
• bumps route-engine-js to 1.45.0 and regenerates localization validators

Companion client PR:

• feat: add audio effect authoring and BGM boundary effects routevn-creator-client#1088

Validation:

• bun run lint
• bun run test (2,031 tests passed)
• bun run build
• targeted audioChannels/sound-boundary-effects.yaml VT capture