py: remote files (clawhdf5.File(url), File.open_url) through File::storage()

The Python bindings could not open a remote file: they parsed through
File::as_bytes() in eight places (path lookups, object headers,
dataspaces, attributes, group listings, the global heap of
variable-length data), which a storage-backed file does not have.

- Every object of a File now shares one handle (src/handle.rs) that
  runs all file access, metadata included, with the GIL released and
  parses through File::storage() and the clawhdf5_format *_in functions.
  Local files take the same path (their storage is the mmap).
- clawhdf5.File(url) opens any scheme://... through
  clawhdf5_remote::storage_for_url (read-only; another mode is a
  ValueError). File.open_url(url, **options) takes the cache and HTTP
  options (block_size, cache_size, headers, retries, timeout,
  allow_full_download, max_full_download, require_validator,
  max_redirects, max_parallel); File.remote_stats gives the block
  cache's counters.
- Default build: plain HTTP only, no C. https (rustls/ring) and
  s3/gcs/azure (aws-lc-rs) are opt-in features of clawhdf5-py, and
  ci-test.sh's no-C check now covers the crate.
- A failed storage read (network error, file changed on the server) is an
  OSError, never KeyError/ValueError and never data; `key in group`
  raises it instead of answering False.

Tests: the read-vs-h5py suite runs locally and over HTTP (1 MiB and
1 KiB blocks) against a range-capable http.server in the test process
(conftest.RangeServer); test_remote.py covers request counts, cache
hits, a server without Range support, a changed file, a server that
hangs up, 16 threads, and a spinning thread that keeps running while a
read waits on 0.2 s requests.

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

crates/clawhdf5-py/tests/test_read_vs_h5py.py

@@ -178,15 +178,32 @@ def _write_fixture(h5py, path):
         g.attrs["depth"] = np.int8(3)
 
 
-@pytest.fixture(scope="module")
-def pair(h5py, tmp_path_factory):
-    path = str(tmp_path_factory.mktemp("h5") / "fixture.h5")
+@pytest.fixture(scope="module", params=["local", "http", "http-1k-blocks"])
+def pair(request, h5py, tmp_path_factory):
+    """The fixture file through h5py and through clawhdf5: opened locally,
+    and over HTTP range requests (a local server in this process) with the
+    default 1 MiB blocks and with 1 KiB blocks, so every structure is read
+    through many small ranges."""
+    from conftest import RangeServer
+
+    root = tmp_path_factory.mktemp("h5")
+    path = str(root / "fixture.h5")
     _write_fixture(h5py, path)
     theirs = h5py.File(path, "r")
-    ours = clawhdf5.File(path, "r")
+    server = None
+    if request.param == "local":
+        ours = clawhdf5.File(path, "r")
+    else:
+        server = RangeServer(root)
+        if request.param == "http":
+            ours = clawhdf5.File(server.url("fixture.h5"))
+        else:
+            ours = clawhdf5.File.open_url(server.url("fixture.h5"), block_size=1024)
     yield ours, theirs, path
     theirs.close()
     ours.close()
+    if server is not None:
+        server.close()
 
 
 def _all_datasets(h5py, f):