AUTHORITATIVE LIMBFORGE MIGRATION CONTRACT
==========================================

projects additions
------------------
oi_type TEXT NOT NULL DEFAULT 'none'
CHECK oi_type IN ('none', 'dual_cone', 'legacy_abutment')

validation
----------
osseointegration = 0  -> oi_type must be 'none'
osseointegration = 1  -> oi_type must be 'dual_cone' or 'legacy_abutment'

measurement_sessions table
--------------------------
id TEXT PRIMARY KEY
project_id TEXT NOT NULL
status TEXT NOT NULL
guide_version TEXT NOT NULL
started_at TEXT NOT NULL
completed_at TEXT
created_at TEXT NOT NULL
updated_at TEXT NOT NULL
FOREIGN KEY(project_id) REFERENCES projects(id) ON DELETE CASCADE

design_forge_measurement_snapshots table
----------------------------------------
id TEXT PRIMARY KEY
project_id TEXT NOT NULL
session_id TEXT
snapshot_json TEXT NOT NULL
snapshot_sha256 TEXT NOT NULL
created_at TEXT NOT NULL
immutable INTEGER NOT NULL DEFAULT 1
FOREIGN KEY(project_id) REFERENCES projects(id) ON DELETE RESTRICT
FOREIGN KEY(session_id) REFERENCES measurement_sessions(id) ON DELETE RESTRICT

required behaviour
------------------
- Existing non-OI records migrate to oi_type='none'.
- OI projects cannot save without a valid OI type.
- Snapshots are append-only and cannot be overwritten.
- Measurement values remain editable until a Design Forge snapshot is created.
- No Forge generation is authorised merely by creating a snapshot.
- Existing project and measurement APIs remain backward compatible.