Adds snapshot system to Helix Stadium:
Backend:
- Snapshot data model (8 slots per preset, captures block bypass
+ params + master volume)
- Snapshot API endpoints: list, save, recall, rename, delete
- Integrated into preset serialization/deserialization
- Tracks current_snapshot in state / WebSocket broadcasts
Frontend:
- Camera icon in header bar with current snapshot number
- SnapshotPanel as slide-up overlay
- 8 snapshot slots in a horizontal row
- Tap to recall filled snapshot, tap empty slot to save
- Press-and-hold (600ms) to save current state
- Inline name editing on tap
- Discard Edits toggle
- Green flash indicator on save
- Delete button for active snapshot
- Add routing_mode (mono/4cm) and routing_breakpoint (default 7) to Preset
- Serialize/deserialize in _preset_to_dict / _preset_from_dict
- Backward compatible — old presets default to mono/7
- Add get_4cm_config() / set_4cm_config() helpers on PresetManager
- set_4cm_config validates mode and saves back to disk
- All 48 related tests pass (46 preset + 2 integration activation)
shawn