docs: plan WAV audio clip import

[britle20]

Summary

• Adds docs/features/15-wav-file-import-as-audio-clip.md for issue WAV file import as audio clip #39.
• Defines the sidebar clip add flow: Build a clip keeps the existing hybrid clip path and Import a file imports a local WAV as an audio clip.
• Documents audio clip metadata, runtime-only imported file data, and the limitation that imported files are not persisted until a future IndexedDB/persistence feature.
• Updates product, architecture, data model, audio engine, UI, testing, README, and PLANS docs.

Key Decisions

• First import format is WAV only.
• Imported WAVs create audio clips, not pitched instruments and not hybrid clips.
• Imported file bytes, File, Blob, object URLs, decoded AudioBuffer, and audio nodes stay out of project JSON.
• Import-time duration adjustment is excluded.
• Future arrangement duration adjustment is feasible via ClipInstance.lengthTicks and optional source offsets, but without time stretching it should mean non-destructive trim/crop or silence extension.

Testing

• git diff --check
• conflict marker search across PLANS.md, README.md, and docs/
• verified new feature spec path exists

Notes / Limitations

• This is a planning PR only and does not implement import UI or audio playback.
• Implementation issue: WAV file import as audio clip #39

Refs #39