Bind a device public key to a machine_id (ADR-0009). The private key never leaves the device (Secure Enclave on Mac); this only ever receives the PUBLIC key + a proof of the right to enroll it. Additive alongside the bearer token during the migration window.

proof.kind:

• "tofu" : first enroll for a machine (no live key yet). First-wins; a second tofu is refused. TOFU is safe because enrollment happens at install, before the machine_id is exposed to any AI.
• "bearer" : migration — the machine's existing bearer token proves ownership; binds the key and supersedes the token as the auth path.
• "sig" : rotation — signed by the CURRENT live key over enroll_message(mid, new_pub); only the old key's holder can swap it, and the old key dies.