clawhdf5-remote: block cache and HTTP range reads (open_url)

Range-read milestone M3, first half: a new crate with the block cache the
design makes mandatory for remote files and an HTTP backend, so
open_url("http://...") gives a clawhdf5::File over File::open_storage.

BlockCache wraps any Storage: aligned blocks (1 MiB by default, the size
docs/design/range-reads.md section 2 measured), LRU with a byte budget,
the missing blocks of one read_at/read_ranges fetched with one backend
read_ranges call as runs of consecutive blocks (a one-block gap filled to
merge runs, each request at most 8 MiB), and reads that miss more than
half the budget not kept. Thread-safe without holding the lock across a
fetch: a block being fetched is in flight, a second reader waits for it
instead of fetching it again, and a failed fetch fails its waiters and is
not cached. A backend holding the file in memory passes through.

HttpStorage (ureq, no TLS by default; `https` adds rustls with ring):
opening is one ranged GET of the first block, whose Content-Range gives
the length (the cache keeps the bytes). The file is pinned by a strong
ETag (If-Match), else Last-Modified (If-Unmodified-Since), and its length,
checked on every response: a change is RemoteError::FileChanged, never
mixed data. A server that ignores Range is refused without reading the
body unless a full download is allowed. Connection errors, timeouts,
408/429/5xx and short bodies are retried with exponential backoff;
Accept-Encoding: identity, and an encoded body is refused. read_ranges
fetches its ranges in parallel.

Tests (a std-only HTTP/1.1 server in tests/common/server.rs, also the
range_server example): every fixture read over HTTP gives File::open's
transcript (CLAWHDF5_REMOTE_CORPUS adds the conformance corpus), with
request counts per file with and without the cache; an h5py-written file
against libhdf5's values; a multi-block file fetched in whole blocks, each
once; a server ignoring Range; a file replaced mid-read (ETag,
Last-Modified, length only); truncated bodies and 503s (retried, then an
error, never cached); a slow server with 8 concurrent readers (no block
fetched twice); bad URLs, 404, encoded bodies, non-HDF5 data. The cache
has unit tests for coalescing, splitting, LRU order, large reads,
failures and concurrent in-flight dedup.

ci-test.sh: clawhdf5-remote joins the no-C default-build check, and its
https feature is linted.

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

crates/clawhdf5-remote/README.md

@@ -0,0 +1,71 @@
+# clawhdf5-remote
+
+Read HDF5 files where they live — on an HTTP(S) server — with
+[clawhdf5](../../README.md), without downloading them first.
+
+```rust
+let file = clawhdf5_remote::open_url("http://127.0.0.1:8000/data.h5")?;
+let temps = file.dataset("/grid/temperature")?.read_f64()?;
+```
+
+The result is an ordinary `clawhdf5::File`: groups, datasets, attributes,
+selections, variable-length data. Only the bytes an operation needs are
+fetched, by `Range` requests, through a block cache.
+
+## How it reads
+
+- **Block cache** (`BlockCache`, mandatory for remote files; see
+  `docs/design/range-reads.md` §2): aligned blocks of 1 MiB by default, LRU
+  with a byte budget (64 MiB by default), the missing blocks of one read
+  fetched as runs of consecutive blocks (a gap of one block is fetched to
+  merge two runs), each request at most 8 MiB, the requests of one read in
+  parallel. A read that misses more than half the budget is not kept, so a
+  big dataset read does not evict the metadata. Readers on several threads
+  share one cache; a block is never fetched twice at once — a second reader
+  waits for the first one's request.
+- **Opening costs one request**: a `GET` of the first block, whose
+  `Content-Range` gives the file's length. HDF5 files keep the superblock
+  and usually the root group's metadata there, so listing a small file
+  often needs nothing more.
+- **Pinned to one version of the file**: a strong `ETag` is sent back as
+  `If-Match` (else `Last-Modified` as `If-Unmodified-Since`) and checked on
+  every response, as is the length. A file replaced while open is an error
+  (`RemoteError::FileChanged`), never a mix of old and new bytes. A server
+  with neither validator can only be checked by length;
+  `HttpOptions::require_validator` refuses it.
+- **Servers that ignore `Range`** (answer `200` with the whole file) are
+  refused (`RemoteError::RangeNotSupported`) without reading the body,
+  unless `HttpOptions::allow_full_download` is set; then the file is
+  downloaded once and read from memory.
+- **Retries**: connection failures, timeouts, `408`/`429`/`5xx` and bodies
+  that end early are retried with exponential backoff (3 retries, from
+  200 ms). Bodies are requested with `Accept-Encoding: identity`; an encoded
+  body is refused.
+
+The zero-copy methods of `clawhdf5` (`read_raw_ref`, `read_*_zerocopy`,
+`File::as_bytes`) borrow the whole file from memory, so they are errors
+(`as_bytes` a panic; use `File::contiguous_bytes`) on a remote file.
+
+## Features
+
+| Feature | What | C code |
+|---|---|---|
+| `http` (default) | `http://` through `ureq`, no TLS | none |
+| `https` | `https://` through rustls, ring provider, Mozilla roots | ring (C and assembly) |
+
+The default build compiles no C (`scripts/ci-test.sh` checks it).
+
+## Counting requests
+
+```rust
+use clawhdf5_remote::{storage_for_url, Options};
+let storage = storage_for_url(url, &Options::default())?;
+let file = clawhdf5::File::open_storage(storage.clone())?;
+// ... read ...
+let s = storage.stats(); // requests, bytes_fetched, hits, misses, cached_bytes, ...
+```
+
+`cargo run -p clawhdf5-remote --example range_server -- DIR` serves a
+directory with range support (the server the tests use), and
+`cargo run -p clawhdf5-remote --example read_url -- URL [DATASET]` lists a
+file and prints what it cost.