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/Cargo.toml

@@ -17,11 +17,20 @@ crate-type = ["cdylib", "rlib"]
 [dependencies]
 clawhdf5_rs = { path = "../clawhdf5", version = "2.7.0", package = "clawhdf5" }
 clawhdf5-format = { path = "../clawhdf5-format", version = "2.7.0" }
+# Remote files (`clawhdf5.File(url)`): plain HTTP by default, which builds
+# no C. HTTPS and the object stores are opt-in features below.
+clawhdf5-remote = { path = "../clawhdf5-remote", version = "2.7.0" }
 pyo3 = "0.29"
 numpy = "0.29"
 
 [features]
 extension-module = ["pyo3/extension-module"]
+# https:// URLs (rustls with ring, which compiles C and assembly).
+https = ["clawhdf5-remote/https"]
+# s3://, gs://, az:// URLs (object_store; its cloud clients build aws-lc-rs, C).
+s3 = ["clawhdf5-remote/s3"]
+gcs = ["clawhdf5-remote/gcs"]
+azure = ["clawhdf5-remote/azure"]
 
 [package.metadata.docs.rs]
 features = []