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]>

README.md

@@ -533,8 +533,20 @@ with clawhdf5.File("data.h5", "r") as f:
 
     records = f["table"]               # compound -> numpy structured array
     ids = records["id"]                # one field
+
+# A file on a web server: range requests through a block cache, nothing
+# downloaded up front; the same read API. The GIL is released while waiting.
+with clawhdf5.File("http://data.example.org/run42.h5") as f:
+    first = f["group/temperatures"][0]
+f = clawhdf5.File.open_url("http://data.example.org/run42.h5", block_size=256 * 1024,
+                           headers={"Authorization": "Bearer ..."})
 ```
 
+The default build reads `http://` URLs only; build with
+`maturin develop --release --features https` (rustls with ring, which
+compiles C) for `https://`, and `--features s3` (or `gcs`, `azure`) for
+object-store URLs.
+
 Reads cover integers and IEEE floats of every width in either byte order,
 `bool`, enums, complex, fixed and variable-length strings, variable-length
 sequences, opaque, HDF5 array types and compounds; other types (references,