[spike] S0.2 Atomic lease API (claim / heartbeat / fence)

[alexfrmn]

Part of #77.
Single SQL contract + thin Node and Python wrappers (coldstart is Python).

• claim_or_skip: INSERT ... ON CONFLICT(conversation_id,member_slot) DO UPDATE ... WHERE stale OR same_session, token=token+1 RETURNING token (CAS, read changes()).
• heartbeat: UPDATE WHERE token/session still match.
• fence: outbound re-checks token==current; else drop/mark superseded.
• heartbeat 5s, TTL 20s, reclaim after 2 missed, jitter.

AC:

• S0.2a concurrency: two processes claim simultaneously -> exactly one wins.
• S0.2b WAL smoke: heartbeat under concurrent daemon+foreground+coldstart, p99 write latency no regression, no DLQ storm.

[alexfrmn]

Shipped in v2.4.0 (PR #92, @murmurv2/core@0.3.0, live-verified).