feat(platform): gate production metadata observability

[zhouning]

Summary

• add a fail-closed production Metadata Fabric observability profile and readiness evaluator
• separate valid-but-incomplete configuration from a fresh, profile-bound protected-environment attestation
• reject placeholders, non-production endpoints, sensitive fields, stale evidence, contract drift, missing owners/SLO/runbook, and production overclaims
• update ADR-044, the AR-0/AR-1 roadmap, the system-of-record matrix, and required CI

Current result

• checked-in profile: profile_valid=true
• production inputs: 20 explicit blockers
• ready_for_protected_verification=false
• production_observability_gate_passed=false
• production_ready=false
• no persistent metrics backend or long-lived component was deployed

Validation

• 532 passed in the Platform Required Tests set
• 11 passed in the focused observability gate suite
• python -m data_agent.metadata_fabric_observability_gate validate
• frontend npm run build
• git diff --check

Stack

Base is M2c-3 / PR #11 (feat/ar1-metadata-fabric-otel-failure-rehearsal). This PR is intentionally not merged.