nx_rate_limit.nx

nx_rate_limit.nx -- token-bucket rate limiter for brute-force resistance. Per docs/SECURITY_POSTURE.md principle P3: every API that verifies a signature, unseals a capability, or processes auth must rate-limit retries from the same source. Without this, an attacker with unlimited tries can exhaust the search space of any K-bit secret in 2^K / rate seconds, and current commodity hardware lets that be very fast. Token bucket semantics: - Each (operation, identity) pair has a bucket of `capacity` tokens, refilled at `refill_per_sec` tokens/second. - Each `consume(op, id)` call removes 1 token; if bucket is empty, the call fails (returns -EAGAIN). - State is persistent across restarts (caller stores the serialized bucket; we provide load/save). Lockout escalation: - After `lockout_threshold` consecutive failures from same identity: lockout_duration grows exponentially (1s, 1min, 1hr, 1day, permanent). - Permanent lockout requires explicit `nx_rl_admin_unlock` by an authority key (multi-person if K-of-N policy is enabled; single-key fallback for early dev). Logging: every failure emits a signed log entry through nx_attest (when shipped) so external witnesses can observe abuse patterns. Pairs with: nx_atom (per-bucket CAS), nx_attest (failure logs), nx_pcc (proof of "bucket invariant: tokens in [0, capacity]").