Add fsstore: test-only reference checkpoint backend

Add a filesystem/JSON reference backend that implements the full api/checkpoint contract, plus a seam integration test that drives a git primary with the fsstore as a configured mirror through checkpoint.Open and asserts all four WriteRequest variants land in both backends while reads resolve from the git primary.

fsstore is registered only via RegisterForTesting (idempotent), never by production code, so a production binary cannot select the "fs" type. It intentionally ignores the contract's git-specific blob-hash fields and stores transcript bytes directly — keeping the example small and making the contract's remaining git leakage concrete. It serves as both the seam's test exercise and a worked example for implementing a new backend.