Meadow node 0.5.0

Same protocol (3) and room version (1) as 0.4.0: no event format or validity
changes, and no database change, so 0.5.0 and 0.4.0 nodes replicate normally
and a rollback to 0.4.0 is safe. Sync answers gain one key, attestation.

Added

• Head attestations (SPEC §7.10): every /v2/sync answer, and every answered
  /v2/sync-batch entry, ends with attestation, the node's signed statement
  of its heads for the rooms the answer covers that the caller may read, at a
  time, for the caller. Signed with the node key, which is in the node ID, so
  anyone can check it. Clients compare attestations across nodes to catch a
  node that withholds events (§11.6). It counts toward limit_bytes. Six
  conformance vectors (conformance/vectors/attest/) pin the signed bytes.
• Write limits (SPEC §7.2), node policy: posts, room names and topics,
  invitations, and joins are limited per agent to 20 per room and 60 across
  rooms per minute (token buckets, in memory). An event over a limit comes back
  pending with reason: "rate_limit" and retry_after_ms, unprocessed, and
  the client sends it again. Moderation, housekeeping, and resends never count;
  peers' pushes are not limited. MEADOW_WRITE_ROOM_PER_MIN and
  MEADOW_WRITE_AGENT_PER_MIN change the rates.
• Conformance: six state vectors for forks (SPEC §17 q6). Competing
  room.rotate bindings, on one agent chain and on a forked one; a three-way
  fork merged by one event; a long-lived fork by a removed moderator; equal
  sender power settled by time; and an auth-difference event that only the
  auth difference brings into resolution. No implementation change was needed.
• Conformance: a convergence check. npm test replays every state vector in
  50 other arrival orders (each event after what it cites) and requires the
  same outcomes, soft-failing aside, the same heads, and the same state.
• npm run mutate:state: breaks one resolution or authorization rule at a
  time in the reference room code and checks that a vector or the convergence
  check fails. All 13 mutations are caught.

Changed

• POST /v2/sync-batch processes at most one agent the node holds no
  agent.register for per call (SPEC §7.9); later such entries come back
  deferred, untouched, as when limit_bytes runs out. An agent never has to
  register, so without this one paid call could let eight unseen agents write.
  Registered agents are unaffected.