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/src/lib.rs

@@ -14,6 +14,7 @@ mod convert;
 mod dataset;
 mod file;
 mod group;
+mod handle;
 mod node;
 mod select;
 
@@ -63,7 +64,7 @@ fn _panic_for_test() -> PyResult<()> {
 /// Convert a `clawhdf5_rs::Error` into a `PyErr`.
 ///
 /// Maps different error variants to more specific Python exception types:
-/// - I/O errors -> `PyIOError`
+/// - I/O errors, and failed reads of a remote file -> `PyIOError`/`PyOSError`
 /// - Format/parsing errors -> `PyValueError`
 /// - Missing dataset/path errors -> `PyKeyError`
 /// - Invalid arguments -> `PyValueError`
@@ -73,6 +74,10 @@ pub(crate) fn to_py_err(e: clawhdf5_rs::Error) -> PyErr {
     use clawhdf5_rs::Error;
     match &e {
         Error::Io(_) => PyErr::new::<pyo3::exceptions::PyIOError, _>(e.to_string()),
+        // A failed read of the storage: a network error on a remote file.
+        Error::Format(clawhdf5_format::error::FormatError::Storage(_)) => {
+            PyErr::new::<pyo3::exceptions::PyOSError, _>(e.to_string())
+        }
         Error::Format(_) => PyErr::new::<pyo3::exceptions::PyValueError, _>(e.to_string()),
         Error::NotADataset(_) | Error::MissingMessage(_) => {
             PyErr::new::<pyo3::exceptions::PyKeyError, _>(e.to_string())