test(frontend): isolate diagram-list readiness before enabling fake timers

[seonghobae]

Test reliability gap

The polluted sanitizeHandleId branches #1007 and #989 contain an unrelated change in frontend/src/App.coverage.test.tsx: four tests click the first diagram 열기 button immediately after opening the diagram panel, then enable fake timers. The candidate patch waits for at least one 열기 button before switching timers.

This finding is not part of handle-ID production performance and must not be carried by #1011.

Required validation

1. Reproduce the predecessor failure or race with repeated focused runs under the repository-required Node 26 runtime.
2. Prove the asynchronous boundary is the diagram-list render, not an unrelated mocked request or stale state leak.
3. Keep waitFor under real timers and enable fake timers only after the list is visible.
4. Preserve every existing polling, undo, terminal-refresh, and error assertion.
5. Run at least 20 focused repetitions plus the complete frontend test, typecheck, coverage, and production build.
6. No production, dependency, lockfile, performance, or Jules-journal change.
7. Record the root cause and exact-head evidence in docs/doctoring/ and the product technical gap baseline only if the race reproduces.

Source disposition

#1007 and #989 mix this candidate test-only stabilization with the duplicate handle-ID optimization. Close those mixed branches after this issue preserves the unique finding.