M12: Extract the first internal shared runtime substrate from the three-engine family #120
Description
Summary
Start the post-M11 micro-extraction roadmap toward an honest internal DB-building library.
Scope
- extract the smallest runtime pieces that are already mechanically shared
- keep command surfaces, snapshot schemas, recovery entry points, and state-machine logic engine-local
- treat this as internal runtime work, not public framework work
Success
- one or more tiny runtime modules are shared cleanly across engines
- behavior remains unchanged
- extraction reduces duplicate maintenance without forcing awkward generic APIs
Canonical docs
- docs/runtime-extraction-roadmap.md
- docs/reservation-runtime-seam-evaluation.md