Phase 1: storage + catalog layer

- storaged: object_store backend (LocalFileSystem), PUT/GET/DELETE/LIST endpoints
- shared: arrow_helpers with Parquet roundtrip + schema fingerprinting (2 tests)
- catalogd: in-memory registry with write-ahead manifest persistence to object storage
- catalogd: POST/GET /datasets, GET /datasets/by-name/{name}
- gateway: wires storaged + catalogd with shared object_store state
- Phase tracker updated: Phase 0 + Phase 1 gates passed

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/storaged/src/backend.rs

@@ -0,0 +1,10 @@
+use object_store::ObjectStore;
+use object_store::local::LocalFileSystem;
+use std::sync::Arc;
+
+/// Initialize the object store backend.
+/// Starts with LocalFileSystem. Swap to S3/RustFS by changing this function.
+pub fn init_local(root: &str) -> Arc<dyn ObjectStore> {
+    std::fs::create_dir_all(root).expect("failed to create storage root");
+    Arc::new(LocalFileSystem::new_with_prefix(root).expect("failed to init local object store"))
+}