Investigate summary-only merge lifecycle concerns

[mchwang]

Context

Copilot's review of PR #46 at validated head 61f5039bf152f36f5548f0a80f6712e5b7269c81 reported Findings: None and confirmed all four prior concrete findings resolved, but its summary stated that five moderate concerns remain across cancellation reasons, cache invalidation, stale errors, and poll ordering. It provided no line locations, scenarios, or inline threads for those concerns.

Investigation scope

For each category, identify a concrete state transition and controlled interleaving before changing code:

• preserve the original timeout/cancellation reason through coordinator, gateway, durable attempt, HTTP response, and UI;
• invalidate status caches after successful and refused irreversible mutations, and reject stale in-flight cache publication;
• prevent stale polling or action errors from overwriting a newer successful UI state;
• prove poll ordering cannot apply an older observation after a newer action, retry, or lifecycle transition;
• identify the fifth concern if it is distinct from the four categories named in the summary.

Acceptance

• Add a failing-before controlled regression for every reproduced issue, then a passing-after fix.
• If a category cannot be reproduced, record the examined paths and evidence rather than patching vague wording.
• Re-run unit, typecheck, and applicable browser race suites.
• Keep this issue non-blocking for PR Complete merge-queue lifecycle and retry guards #46 unless a concrete safety failure is reproduced against its validated head.