Publish @stateproof/core as an installable package

[SurefireStudios]

packages/core (schemas, assertion DSL and evaluator, state diff, evidence references, metrics, replay, canonical serialization) has no dependency on the benchmark or the agents and is the part other projects would reuse.

Done when

• A build step emits ESM plus type declarations.
• The package is published under the @stateproof scope with the assertion schema version in its changelog.
• The Release workflow publishes on a v* tag with provenance.