gen: stamp the codegen-ts engine version into .gen-state and warn on mismatch #232

Problem

.metaobjects/.gen-state/ records per-file content hashes only — not the @metaobjectsdev/codegen-ts engine version that produced them. Because scaffold-and-own (ADR-0034) copies only thin generator wrappers while the render* primitives + defaults stay in the package, a consumer who runs npm update && meta gen after an engine change can get a surprising diff (or a three-way-merge conflict) with no signal about why the output moved.

This is the mitigation identified in the versioning-policy analysis: under the (correct) pre-1.0 policy, output-changing bugfixes ship as PATCHes and auto-flow through ^ ranges — so the safety net is making every such diff explained, not slowing the cadence.

Proposal

1. When meta gen writes the gen-state snapshot, record the resolved @metaobjectsdev/codegen-ts version alongside the content hashes.
2. On the next meta gen, if the recorded engine version differs from the currently-installed one, print one informational line before writing, e.g.:
   meta gen: codegen engine 0.19.3 → 0.20.1 since last gen — generated output may differ; see CHANGELOG.
3. Purely informational (never blocks); complements the existing per-file merged/conflict/unchanged statuses.

Pointers

• gen-state / overwrite policy: server/typescript/packages/codegen-ts/src/overwrite-policy.ts, runner.ts
• gen status surfacing: server/typescript/packages/cli/src/commands/gen.ts
• policy context: docs/RELEASING.md → "Versioning policy (pre-1.0)"

Follow-up from the versioning-cadence review (2026-07-25).