Cutover and canary rollout

[aram356]

Description

Deploy EdgeZero-backed Fastly entry point behind edgezero_enabled config flag. Run canary traffic progression: 1% → 10% → 50% → 100% with hold points. Validate rollback by setting edgezero_rollout_pct to 0.

Done when

• Config flag live
• Canary at 1% → 10% → 50% → 100% with hold points
• Rollback tested

Blocked by

Phase 5: Verification gates

References

• docs/internal/EDGEZERO_MIGRATION.md — Phase 5: Cutover plan
• Part of EdgeZero migration #480