docs: range-read milestone M3 — remote files

README: "Reading remote files" (open_url, the range_server and read_url
examples with their real output against the fixtures, h5rs on URLs), the
crate in the crate map and the unreleased highlights. CHANGELOG: the
clawhdf5-remote crate, h5rs URLs, File::storage and
VlResolver::element_in, with the request counts over the conformance
corpus (tank, 2026-09-26, the command given). known-issues: the M2
range-read entry updated (the cache now exists; h5rs reads through
storage) and a new entry for the remote backends' limits (no Python or
browser URLs yet, fixed block size, cloud stores not run against a real
bucket, validators, credentials). Design doc: M3 status with the choices
that differ from the plan (a crate rather than a clawhdf5-io feature,
ureq for HTTP so the default build has no C) and the corpus counts.

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

README.md

@@ -96,6 +96,12 @@ breaking change, are in [CHANGELOG.md](CHANGELOG.md).
   filtered top-k, never slower than unfiltered), and opt-in re-ranking and
   confidence rejection, which used to be reachable only through `ClawhdfBackend`.
 
+**Remote files (unreleased)**
+- New crate `clawhdf5-remote`: `open_url("http://…")` reads a file on an
+  HTTP server (or in S3/GCS/Azure, opt-in) by range requests through a
+  block cache, without downloading it; `h5rs` takes URLs with its `remote`
+  feature. See [Reading remote files](#reading-remote-files).
+
 **Tooling**
 - CI now runs the h5py/netCDF4 interop suites for real (they had been skipping
   silently) and runs an aarch64 job for the NEON kernels.
@@ -450,6 +456,50 @@ Each call changes the file in place (no rewrite) and syncs it. What it
 cannot change safely is refused before anything is written; see
 [known issues](docs/known-issues.md) for the limits.
 
+### Reading remote files
+
+[`clawhdf5-remote`](crates/clawhdf5-remote/README.md) opens a file on an
+HTTP server (or, with its `s3`/`gcs`/`azure` features, in an object store)
+without downloading it: the read API is the same `clawhdf5::File`, and
+only the bytes an operation needs are fetched, by `Range` requests through
+a block cache (1 MiB blocks; opening fetches the first one). A file that
+changes on the server while it is open is an error, never a mix of old and
+new bytes.
+
+```rust
+let file = clawhdf5_remote::open_url("http://127.0.0.1:8000/tall.h5")?;
+let values = file.dataset("/g2/dset2.1")?.read_f64()?;
+```
+
+To try it without a server of your own, the crate's test server serves a
+directory with range support:
+
+```bash
+cargo run -p clawhdf5-remote --example range_server -- crates/clawhdf5/tests/fixtures 127.0.0.1:8000
+# in another shell: list the file, read one dataset, print what it cost
+cargo run -p clawhdf5-remote --example read_url -- http://127.0.0.1:8000/tall.h5 /g2/dset2.1
+```
+
+```text
+/g1  group
+/g2  group
+/g2/dset2.1  dataset [10] F32
+/g2/dset2.2  dataset [3, 5] F32
+/g1/g1.1  group
+/g1/g1.2  group
+/g1/g1.2/g1.2.1  group
+/g1/g1.1/dset1.1.1  dataset [10, 10] I32
+/g1/g1.1/dset1.1.2  dataset [20] I32
+/g2/dset2.1: 10 values, first [1.0, 1.100000023841858, 1.2000000476837158, ...]
+1 range requests (the one at open included), 9968 bytes fetched, 9968 bytes cached
+```
+
+(`tall.h5` is 9 968 bytes, so the first block holds all of it.) `h5rs`
+built with `--features remote` takes the same URLs:
+`h5rs ls -r http://127.0.0.1:8000/tall.h5`. Plain HTTP builds no C;
+`https://` is the `https` feature (rustls with ring, which compiles C).
+Limits are in [known issues](docs/known-issues.md).
+
 ### Python
 
 `crates/clawhdf5-py` is a Python package (PyO3 + numpy) that reads HDF5 with
@@ -678,7 +728,7 @@ let exported = backend.export_markdown("MEMORY.md")?;
 ## Crate Map
 
 ```
-clawhdf5 workspace (17 crates, ~86K lines of Rust in src/, ~104K with tests
+clawhdf5 workspace (19 crates, ~86K lines of Rust in src/, ~104K with tests
                      and benches; plus libaec-sys, an internal FFI bindings
                      crate for the optional szip feature)
 │
@@ -690,7 +740,8 @@ clawhdf5 workspace (17 crates, ~86K lines of Rust in src/, ~104K with tests
 │   ├── clawhdf5             — High-level API
 │   ├── clawhdf5-netcdf4     — NetCDF-4 support
 │   ├── clawhdf5-accel       — SIMD (AVX2, NEON incl. SDOT int8; AVX-512 behind `avx512`)
-│   └── clawhdf5-gpu         — GPU compute (wgpu, hand-written WGSL compute shaders)
+│   ├── clawhdf5-gpu         — GPU compute (wgpu, hand-written WGSL compute shaders)
+│   └── clawhdf5-remote      — Remote files: HTTP(S) range requests, object stores, block cache
 │
 ├── Agent Memory
 │   ├── clawhdf5-agent       — Memory engine (24.7K lines, 32 modules; chained-CRC WAL)
@@ -705,6 +756,7 @@ clawhdf5 workspace (17 crates, ~86K lines of Rust in src/, ~104K with tests
 │   └── clawhdf5-wasm        — Browser (WebAssembly, wasm-bindgen; read-only)
 │
 └── Tooling
+    ├── clawhdf5-tools       — h5rs: ls, dump, stat, diff, check
     └── clawhdf5-bench       — Benchmark suite
 ```