EPIC-0460: publish durable V2 clarifications

[epowell40]

Summary

• materialize authenticated missing-input clarifications as durable Assignment Kernel V2 events
• publish the exact V2 outcome on early and normal Codex response paths
• make clarification responses atomic, restart-idempotent, and conflict-safe
• keep V1 outcome projection isolated to historical assignments

Validation

• operator-backend: 2,891 passed, 0 failed, 3 standalone skips
• MCP server: 218 passed, 0 failed
• native certification: net48/net8 passed and regenerated
• Assignment Kernel boundary, benchmark runtime boundary, and module-size ratchets passed
• Candidate 10 missing-input replay and V2 handoff regressions passed