App-mount race on fast/cold opens: first capture can see an unreadable or empty tree (cross-platform)

Found during #1555's post-merge evidence round (artifacts /private/tmp/ad-p5-evidence-FINAL/retry2/). Two independent sightings of the same shape on DIFFERENT platforms in one session, distinct from #1569's scroll-specific mechanism:

- **iOS**: replay failed at step 2 (`react-native dismiss-overlay`) with "accessibility tree is unreadable" on a lightly-loaded host (~4.0), immediately after a fast session-reused `open` (`postOpenSettleDurationMs:300`). Artifacts: retry2/leg1-ios-run1-retry.
- **Android**: the FIRST replay of a freshly-recorded script diverged at its first click with a fully empty snapshot (`refs:[]`) immediately after a cold `open --relaunch`; an immediate retry went green. Artifacts: retry2/leg3-extended (item b, first attempt).

Both look like the app not yet mounted/AX-registered when the first post-open capture lands — the open-settle window is closing before the app's tree is actually readable. Suggested investigation: what does post-open settle currently wait on per platform, and should the first capture after open carry a readable/non-empty-tree precondition with a bounded retry (attempt-bound per the #1569 lesson, not wall-clock)?

Related: #1569 (distinct mechanism, same corpus), the #1105 capture-wedge lineage (iOS), #1254/#1259 (Android a11y-cache freshness — the helper clears cache per capture, so the Android sighting is likely mount-timing, not cache).

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