feat: framework-agnostic core — language-neutral manifest + engine

[jaylann]

The data map is already a serializable, versioned manifest. Explore formalizing it as a language-neutral format so engines in other languages (TS, Go, …) become thin authoring layers over the same semantics instead of rewrites. Interacts directly with the MANIFEST_SCHEMA_VERSION forward-migration guarantee and widened SemVer — this is ADR territory before any code. Only worth doing after one stack is fully proven.

Exploring: scope notes, no commitment, no order. Related: #66 (multi-app aggregation), #62 (second adapter).

[jaylann]

#71 is the concrete deliverable here: EffacedStack.from_manifest(session_factory, engine, manifest_payload) (PR #150) runs every engine from a serialized/authored manifest paired with a reflected live database, proven byte-identical to the annotation-driven from_base path by a property test over generated schemas. That establishes the Python side of the storage-agnostic-core promise — a non-Python service can export its manifest and connect effaced to the same database.

#63 stays open for the remaining language engines (TypeScript/Go), tracked by the #75 ADR.