Add scene-scoped voice resources

[han4wluc]

Summary

• add scene-grouped resources.voices and voice action resourceId schema
• resolve voice playback from the active scene with runtime sound volume, mute, loop, and delay handling
• document voice resources and add schema/render/runtime coverage

Tests

• bun run prettier docs/Concepts.md docs/RouteEngine.md spec/RouteEngine.voice.test.js spec/projectDataSchema.test.js spec/system/renderState/addVoice.spec.yaml src/schemas/presentationActions.yaml src/schemas/projectData/resources.yaml src/schemas/projectData/story.yaml src/stores/constructRenderState.js vt/specs/complete/one.yaml -c
• bun run lint
• bun run test