Implement comprehensive AT Protocol support in OCaml with 11 packages:

- atproto-syntax: Identifier parsing (handle, DID, NSID, TID, AT-URI, etc.)
- atproto-crypto: P-256/K-256 cryptography, did:key, JWT
- atproto-multibase: Base32, Base58btc encoding
- atproto-ipld: DAG-CBOR, CIDs, CAR files, blobs
- atproto-mst: Merkle Search Tree implementation
- atproto-repo: Repository operations and commits
- atproto-identity: DID and handle resolution
- atproto-xrpc: HTTP API client/server, OAuth
- atproto-sync: Firehose and repository synchronization
- atproto-lexicon: Schema language parser, validator, codegen
- atproto-api: High-level client API with rich text

All 272 tests pass, covering all 42 fixture files from atproto-interop-tests.
Uses OCaml 5.4 effects for I/O abstraction.