[spike] S0.7 Acceptance harness

[alexfrmn]

Part of #77. Spike exit criteria.

• AC-1 two sessions + all paths live -> exactly 1 reply.
• AC-2 owner crash -> TTL reclaim -> exactly 1 reply.
• AC-3 resurrect race: stale owner resurrects mid-turn -> stale-token outbound suppressed (exactly 1).
• AC-4 concurrent claim -> exactly one wins.
• AC-5 wake routes to owner thread, not new sidecar.
• AC-6 muted observer history preserved.
• AC-7 legacy message without channelId unchanged (broadcast).
• AC-8 WAL smoke (from S0.2b).
  Log owner_token/owner_session/claim_result on all paths.

[alexfrmn]

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