KG versioning — snapshots, branches, and remote sync.

Implements the versioning model from ADR-015 and the implementation details from ADR-042: content-addressed snapshots, named branch pointers, and the remote push/pull protocol.

Crate layout

• [types] — KgSnapshot, KgBranch, SnapshotId, RemoteConfig
• [hash] — canonical JSON serialization + SHA-256 snapshot hashing
• [snapshot] — commit() operation
• [branch] — branch(), checkout(), list_branches()
• [log] — log() operation (snapshot history)
• [merge_engine] — MergeEngine trait + NoOpMergeEngine
• [error] — VcsError type