Fix Rescue forwarding and progress compatibility

[vitry]

Summary

• accept authenticated forwarding-child prompt hooks neutrally without minting parent caller or review-gate authority
• keep named and generic Rescue forwarders attached to one yielded execution handle through terminal exit, with strict choice/cleanup qualification
• make conversation compatibility observable through bounded owner-only structural probe facts and fixed degraded diagnostics
• add a bounded, privacy-safe accepted-current-turn session snapshot fallback with heartbeat cadence, online recovery, and separate authoritative result reads

Safety and privacy

• no raw conversation frames, session logs, assistant prose/reasoning, tool input/output, file contents, paths, identifiers, or capabilities are rendered by snapshot fallback
• progress remains observational and cannot replace the authoritative Rescue result
• installed cleanup revalidates exact PID, stable start identity, parent, and a 256-bit live nonce before every signal

Verification

• npm run check
  • main: 1242 passed, 0 failed, 3 documented opt-in skips
  • marketplace build: 2 passed, 0 failed
  • qualified E2E: 18 passed, 0 failed, 3 documented opt-in skips
• SHA-pinned marketplace build at 5acea20f20a5696fb1003561b8ff43b991512d54
• final independent whole-branch review: Ready for PR, no Critical or Important findings
• formerly flaky snapshot integration: 10/10 concurrent repetitions (30 scenario executions)

The skipped authenticated cases remain explicitly opt-in because they spend external Codex/ZCode credentials or require a real macOS installation:

• ZCODE_CODEX_SKILLS_E2E=1
• ZCODE_CODEX_RESCUE_E2E=1
• ZCODE_REAL_E2E=1

Closes #24
Closes #25
Closes #26
Closes #27