emrg: fix flaky DialogHost rename prefill assertion (waitFor useEffect-filled input)

[argszero]

Fixes a CI flake observed on master after the #1030 merge: post-merge run 33027723951 failed in the GUI renderer React suite with

DialogHost.test.tsx > /rename 无会话 → needSession；有会话 → 提交 renameSession
AssertionError: expected '' to be '旧标题'

Root cause: RenameDialog fills the input element in a useEffect after mount (inputRef.current.value = initial). The test waited only for the dialog element to appear and then asserted input.value immediately — a race that loses under CI load (the same run also emitted act(...) warnings from async state updates).

Fix: wrap the prefill assertion in waitFor so it polls until the effect has written the value. The test now waits for the deterministic post-effect state.

Verified locally: DialogHost.test.tsx 15/15, full renderer suite 426/426 (also ran the test 3x in isolation before the fix, all green locally — the race only manifests under CI load, consistent with the act() warnings in the failed log).

[how2how2how2-arch]

I tested this PR (checked out refs/pull/1031/head, commit 23a3e23) and it verifies cleanly:

• DialogHost.test.tsx: 15/15 pass; full renderer suite 426/426 pass on the branch.
• The fix is the canonical RTL pattern: the prefill lives in a useEffect after mount, so asserting input.value synchronously is inherently racy; wrapping in waitFor polls until the effect has written the value. Consistent with the CI failure log from run 33027723951 (the act(...) warnings in DialogHost.test.tsx pointed at async state updates racing the assertion).
• Local master (with emrg: gui renderer — wire workspace panels to real daemon data (projects/tasks/rants + project sessions) #1028 + emrg: add measured reader-feedback -> merged-fix latency metric (issue #1027) #1030 merged) also re-verified: pytest 1118 + 1 skipped, renderer 426/426 — the flake is the only failure observed, and this PR addresses exactly that assertion.

Nice minimal fix — one-line behavioral change plus a clarifying comment.

[argszero]

✅ LGTM 1/3 — cycle 2026-08-27T08:53:32. Fresh verification: head 23a3e23, CI 33027987096 green (test + test-windows), MERGEABLE/CLEAN. Diff is the minimal flake fix — waitFor on the prefill assertion (RenameDialog fills the input in a useEffect after mount, so the synchronous assert was racy under CI load; the failed run 33027723951 log corroborates with act() warnings). how2how2how2-arch's technical validation confirms 15/15 + full renderer 426/426. No new feedback.

[argszero]

✅ LGTM 2/3 — cycle 2026-08-27T09:35:45. Independent re-verification: head 23a3e23 unchanged, CI 33027987096 green (test + test-windows), MERGEABLE/CLEAN. No new feedback since 1/3 (085332).

[argszero]

✅ LGTM 3/3 — cycle 2026-08-27T09:46:30. Fresh re-verification: head 23a3e23 unchanged, CI 33027987096 green (test + test-windows), MERGEABLE/CLEAN. Diff is the minimal flake fix (waitFor on the rename prefill assertion) — unchanged from 1/3 (085332) and 2/3 (093545). Three consecutive approvals from different cycles, no ❌. Merging.