Restore isolated reader cleanup and fallback

[8Dionysus]

Summary

• Fix isolated search-reader failure/exit cleanup so child cleanup verification and parent-owned fallback remain reachable.
• Keep the required fallback behavior; do not delete it.
• Add failure x cleanup regression coverage.

Validation

• py_compile scripts/aoa_session_memory.py: passed.
• Focused source/identity/portable copy-install checks: 18 passed, 1110 deselected.
• Full static2 owner route: 1312/1312 passed, coverage_complete=true, failed_nodeids=[], skipped_nodeids=[].
• Manual checkpoint review: agent-review:e85635bf:20260908T000310Z.

Limits

This PR contains portable source/test changes only. Installed bundle identity, hook/runtime activation, and semantic acceptance are separate owner-controlled steps after merge.