clawhdf5-remote: object stores through object_store (S3, GCS, Azure)

ObjectStoreStorage (feature `object-store`, pure Rust) reads one object
of any object_store store by ranged get_opts, pinned at open by a head
request: If-Match with its ETag (and the ETag and size of every response
compared), else its version or modification time. A change is
RemoteError::FileChanged. object_store is async and Storage is not, so
the storage owns a small multi-threaded tokio runtime (two workers) and
blocks the calling thread on it; the ranges of one read_ranges call are
fetched concurrently (up to 8). From inside another tokio runtime it
refuses with RemoteError::Usage instead of blocking a worker, and it
shuts its runtime down in the background on drop so dropping it in async
code does not panic.

open_object(store, path, options) opens a file through a block cache
(first block prefetched); open_url accepts s3://, gs:// and az:// with
the `s3`, `gcs` and `azure` features, configured from the environment by
object_store's from_env builders. Those pull object_store's cloud clients
and aws-lc-rs (C), so they are opt-in; without them the URL is a clean
UnsupportedScheme error naming the feature.

Tests against object_store's in-memory and local-file stores (no cloud):
every fixture's transcript equals File::open's, a multi-block object is
fetched in coalesced block runs, an object replaced while open is an
error, and a missing object or a read from inside a runtime is a clean
error. ci-test.sh lints all backends, runs these tests (with s3 for its
URL parsing test) and checks object-store for C in the no-C step.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

CLAUDE.md

@@ -26,7 +26,7 @@ Cargo workspace with 19 crates under `crates/` (plus `libaec-sys`, an internal F
 | `clawhdf5-napi` | Node.js native addon bindings |
 | `clawhdf5-py` | PyO3 Python bindings |
 | `clawhdf5-wasm` | WebAssembly (wasm-bindgen) reader for the browser; demo in `examples/wasm-viewer/` |
-| `clawhdf5-remote` | Remote files: `open_url` over HTTP(S) range requests through a mandatory block cache (`BlockCache`) |
+| `clawhdf5-remote` | Remote files: `open_url` over HTTP(S) range requests and object stores (`object_store`: S3, GCS, Azure) through a mandatory block cache (`BlockCache`) |
 | `clawhdf5-bench` | Benchmark suite |
 
 ## Key Features
@@ -165,8 +165,11 @@ Cargo workspace with 19 crates under `crates/` (plus `libaec-sys`, an internal F
   runs coalesced into parallel requests). `HttpStorage` pins the file by
   ETag/Last-Modified and length (a change is `RemoteError::FileChanged`),
   refuses servers that ignore `Range` unless a full download is allowed,
-  and retries transient failures. Default build is plain HTTP with no C;
-  `https` (rustls + ring) is opt-in. Tests run a std-only HTTP server
+  and retries transient failures. `ObjectStoreStorage` (feature
+  `object-store`, pure Rust) blocks on a small owned tokio runtime and
+  refuses to run inside another runtime. Default build is plain HTTP with
+  no C; `https` (rustls + ring) and `s3`/`gcs`/`azure` (aws-lc-rs) are
+  opt-in. Tests run a std-only HTTP server
   (`tests/common/server.rs`, also the `range_server` example);
   `CLAWHDF5_REMOTE_CORPUS=conformance/.cache/corpus` compares every corpus
   file over HTTP with `File::open`.