M13-T01: Write the internal runtime-vs-engine contract

[skel84]

Summary

Write the internal contract that defines the stable boundary between shared runtime substrate and engine-local semantics.

Scope

• define the responsibilities of the shared runtime
• define the responsibilities that stay local to each engine
• explicitly rule out premature generic state-machine and schema abstractions

Acceptance Criteria

• one focused contract doc is added under docs/
• the contract names the shared modules and the non-shared engine surfaces
• future engine work can cite this document as the authoring boundary

Canonical Docs

• docs/runtime-extraction-roadmap.md

Blocked By

• M13: Define the internal engine authoring boundary after the first extractions #125