rcan 3.7.0

Added

• rcan.assurance: optional recovery fields from rcan-spec#232 (Appendix C.1.3).
  Envelope.stop.safe_state, Envelope.recovery (on_reject, repeated_stops,
  estop_reset), GateDecision.stop_kind and GateDecision.reason_returned, with
  check_envelope_recovery and check_stop_fields for the schema's conditional
  rules. Not a full schema validation.
• BE_LEVELS (BE-1, BE-2, BE-3), the level names schemas/envelope.json has
  used since rcan-spec#226. AssuranceLevel accepts both; ASSURANCE_LEVELS keeps
  its A1-A3 value.
• STOP_KINDS, ON_REJECT_BEHAVIOURS.
• A LICENSE file (MIT, as the package metadata already declared).

Fixed

• NodeClient.resolve() fallback requested {api_base}/api/v1/robots/{rrn}, doubling
  /api/v1 on nodes whose api_base already ends in it (rcan.dev does). It now requests
  {api_base}/robots/{rrn}, as the federation protocol defines.
• README quickstarts call APIs that exist, and were checked against PyPI.

Changed

• Test fixtures re-copied from rcan-spec master (BE levels; new legged-recovery
  envelope and chain, estop-auto-restart invalid envelope).