Serialize ACP terminal transitions

[yyjeqhc]

Follow-up to #129, which merged while the final review was in progress.

Summary

• serialize terminal state commits for each CodingAgentRun
• preserve first-terminal-wins semantics when pre-prompt cancellation races setup failure
• keep the in-memory snapshot, durable record, and terminal event consistent
• add a concurrent regression test covering both possible terminal outcomes

Validation

• cargo fmt -- --check
• cargo test 'coding_agent' --all-targets -p webcodex-runner — 31 passed, 1 ignored
• cargo test --all-targets -p webcodex-runner — 774 passed, 4 ignored (same patch before rebasing onto merged main)
• cargo test 'coding_agent' --all-targets -p webcodex-core — 5 passed
• cargo test 'coding_agent' --all-targets -p webcodex — 18 passed