fix: preserve Antigravity lifecycle custody

[yuezengwu]

Follow-up repair for #2375.

The reviewed head cbe3f0a35068ffedc4ec1066534f6f43169d8fb7 moved to c7a9c03 while this repair was being prepared. To avoid overwriting the external author branch, this PR is stacked from that exact live #2375 head and carries one focused commit.

What changed:

• make provider-entered settlement authority monotonic across overlapping suspend and shutdown calls, so route retirement cannot revoke an operator or full-drain settlement grant;
• return the exact provider conversation ID when lifecycle settlement interrupts a resume that began from a synthetic pending ID;
• add deterministic coverage for operator suspend, full shutdown, concurrency preemption, route retirement, overlapping lifecycle calls, late stdout fencing, exact-ID recovery, and no replay of a mutating first turn.

Validation:

• pnpm check: passed with only existing repository warnings and infos;
• pnpm typecheck: 9 of 9 tasks passed;
• Antigravity plus client capability tests: 110 passed;
• focused Pi handler custody tests: 4 passed;
• focused Pi SessionRuntime custody tests: 7 passed;
• CLI capability refresh tests: 22 passed;
• shared runtime catalog tests: 9 passed;
• sequential shared, client, and CLI builds: passed;
• git diff --check: passed.

A full monorepo test attempt also exposed two unrelated flaky cases in the server durable-ack wake-loss test and the Grok silent-child test; each passed immediately when rerun standalone. No out-of-scope files were changed.

Hard stop retained: the Antigravity CLI is absent on this host, so no live Google authentication, provider start/resume, real tool/MCP/Skills execution, or process-lifecycle validation was attempted. External CLI identity, credentials, permissions, supply-chain behavior, and dangerously-skip-permissions remain gated on full-isolation QA and explicit yuezengwu acceptance. This PR is not merge, release, or deployment authorization.