Move heartbeat/liveness logic into core behind the remote-adapter feature flag #767

Part of #727.

Move the duplicated heartbeat/liveness logic from the postgres and mongodb
adapters into socketioxide_core behind the existing remote-adapter feature
flag, so it can be shared by each adapter.

Current state:

• socketioxide-postgres and socketioxide-mongodb each define the same
  nodes_liveness: Mutex<Vec<(Uid, Instant)>> field, the same interval-based
  heartbeat_job, the same emit_heartbeat/emit_init_heartbeat helpers, the
  same recv_heartbeat handler (~130 LOC duplicated verbatim, only the error
  type differs) and the same server_count dead-node pruning.
• The wire protocol already lives in core: RequestTypeOut/In::Heartbeat (20)
  and InitHeartbeat (21) in crates/socketioxide-core/src/adapter/remote_packet.rs.
• Redis is unaffected: it has no heartbeat (liveness is pub/sub subscriber
  counting via num_serv).

Proposed shared API (sketch, in core behind remote-adapter):

• HeartbeatTracker: wraps nodes_liveness + hb_timeout; on_heartbeat(origin)
  (update-or-push, returns whether an InitHeartbeat reply is due), server_count()
  (prune dead nodes + self), is_alive(uid).
• HeartbeatSender trait: uid(), send_req(...), with default
  emit_heartbeat/emit_init_heartbeat implementations.
• heartbeat_loop(interval, emit) helper and a default recv_heartbeat.

Postgres and mongodb then keep only a thin adapter: a HeartbeatTracker field
plus a small HeartbeatSender impl. No behavior or wire-format change.