test(herdr): stabilize focus-flash E2E sampling

[quinnbot-ai]

Summary

• Sample the operation's exact pre-operation tab rather than global focus.
• Fail immediately on a valid unfocused sample while retrying only unreadable or identity-mismatched reads.
• Retain corrective-action, tab-restoration, and pane-death assertions.

Validation

• no-mistakes review, test, documentation, lint, and push steps passed.
• Ten consecutive busy-fleet E2E runs passed.
• A temporary local mitigation inversion failed the regression as expected.

Pipeline

Updates from git push no-mistakes