investigate(ios): confirm historical XCTest tap-failure storm root cause #1599

Purpose

Confirm or falsify the remaining historical root-cause hypothesis for the iOS XCTEST_RECORDED_FAILURE tap storm seen on hostile/profile-class screens.

The user-facing retry-spiral defect is already fixed:

• fix(ios): corroborate recorded tap outcomes #1605 corroborates ambiguous recorded tap failures against a fresh, comparable post-action capture and returns success-with-warning when the tap landed.
• fix(ios): pin tap-outcome corroboration probes to the baseline's backend #1634 keeps corroboration on the baseline snapshot backend, covering the private-AX/tree penalty-boundary mismatch seen in the live Bluesky repro.
• After perf(ios): halve hostile-screen capture cost under the XCTest-channel penalty #1587, repeated live attempts could no longer reproduce the natural failure storm.

This issue no longer owns tap-outcome recovery, generic runner restart behavior, or additional retry logic.

Remaining question

Was the archived failure storm caused by XCTest-channel saturation and abandoned capture work before #1587, or does an independent current defect remain?

Investigation contract

1. Reproduce against the archived AppControlBench bsky-24 evidence or a controlled pre-perf(ios): halve hostile-screen capture cost under the XCTest-channel penalty #1587 build using the same profile-screen action sequence.
2. Compare a pre-perf(ios): halve hostile-screen capture cost under the XCTest-channel penalty #1587 run with current main under the same app state and command sequence.
3. Retain runner logs and request diagnostics covering:
   • capture duration and penalty state;
   • abandoned or coalesced runner work;
   • XCTEST_RECORDED_FAILURE timing;
   • restart/relaunch boundaries;
   • whether the UI changed despite the reported failure.
4. Distinguish causal saturation evidence from correlation. A changing timeout/failure set or a non-equivalent app state is not confirmation.

Completion

• Confirmed historical cause: record the matched evidence showing the storm disappears when the perf(ios): halve hostile-screen capture cost under the XCTest-channel penalty #1587 channel-pressure change is applied, then close this issue.
• Independent current defect: provide a reproducible current-main sequence and open a narrowly scoped implementation issue for its owning boundary; do not expand this historical investigation into a speculative fix.
• Not reproducible: record the bounded attempts and evidence limits, then close as superseded by perf(ios): halve hostile-screen capture cost under the XCTest-channel penalty #1587, fix(ios): corroborate recorded tap outcomes #1605, and fix(ios): pin tap-outcome corroboration probes to the baseline's backend #1634.

No production change is required unless a distinct current-main defect is reproduced.