[codex] Add mledb to sprocket migration guardrails

[gankoji]

Summary

Adds an auditable guardrail toolchain for beginning the MLEDB schema to Sprocket schema migration.

• adds a migration runner with preflight, coverage, baseline, legacy backfill, validation, and rollback steps
• adds a guard schema that snapshots existing Sprocket rows and detects changed or deleted pre-existing data after a run
• adds semantic validation for the MLEDB domains currently handled by the legacy migration SQL
• adds a domain coverage manifest so deferred MLEDB tables are explicit instead of implied as migrated
• documents the rehearsal runbook, validation gates, error handling, rollback plan, and promotion criteria

Validation

• git diff --cached --check
• bash -n scripts/migration/run-mledb-sprocket-migration.sh
• scripts/migration/run-mledb-sprocket-migration.sh --help
• local disposable Postgres fixture executed preflight, coverage, baseline, and validate with zero validation issues

Notes

This is tooling for migration rehearsal and review. It has not been run against a production dump yet; the plan requires a clean disposable-database rehearsal before production use.