irmin: restructure into irmin/lib/<backend>/, add Irmin.SHA{1,256}, implement diff/ddiff, rewrite all backends + bin for Schema API

- Move backends into irmin/lib/{git,json,cbor,tar,atproto,oci}/
- Rename parse→dec, serialize→enc across Schema API
- Add Irmin.SHA1, Irmin.SHA256 pre-built schema instances
- Deduplicate Schema.Make boilerplate in json, cbor, tar, oci
- Implement structural diff and 4-point ddiff (was stubs)
- Implement JSON serialize via Jsont, CBOR serialize via Cbort
- Rewrite ATProto backend: Schema.Make + Heap.BACKEND + MST bridge
- Rewrite OCI backend: SHA256 JSON schema
- Rewrite all 13 bin commands for Schema/Heap API
- Fix test infrastructure: restrict old tests, rewrite mst_proof
- Fix ocaml-scitt for dec/enc rename
- Two-phase merge API: cursor * conflict list (not Ok/Error)
- Irmin.Merge module with typed combinators + v/v_result lifters
- 11 tests pass (6 schema + 5 tar), mst_proof verified