docs: update INTEGRATION_PLAN with BF16 superposition architecture

- Mark step_handler (1.2) and subsystem_impl (1.3) as DONE
- Add BF16 Superposition Architecture section (hot/cold/feedback loops)
- Document CAM = address IS content (emergent dimension triples)
- Add 256Kbit container architecture (4-container future format)
- Add BindSpace as hub diagram with DeltaLayer/LayerStack overlay
- Add rustynum-core ↔ ladybug-rs type mapping table
- Update key files reference with all cognitive + BF16 files
- Add commit history tracking

https://claude.ai/code/session_016HQG2eoyNAwML6WP8WhbAG

Author: claude