EPIC-0460: return canonical terminal V2 results idempotently

[epowell40]

Summary

• stop V2 progress mutation once terminal truth already exists
• derive a typed, read-only terminal result from admitted domain evidence
• return the same result on turn-stop, reconnect, and duplicate retrieval
• retain strict terminal immutability internally

Validation

• backend: 2,882 tests; 2,879 passed, 3 skipped, 0 failed
• MCP: 211/211
• architecture, benchmark-boundary, and module-size gates passed
• generated certification and exposure evidence refreshed and validated