Add multi-quality variant support for video content:
- New ABR types in pkg/smsg/types.go (ABRManifest, Variant, ABRPresets)
- New pkg/smsg/abr.go with manifest read/write and bandwidth estimation
- New cmd/mkdemo-abr CLI tool for creating ABR variant sets via ffmpeg
- WASM parseABRManifest and selectVariant functions
- Demo page "Adaptive Quality" tab with ABR player
- RFC-001 Section 3.7 documenting ABR format and algorithm
