docs: range-read M4 (openUrl in the browser): changelog, limits, design status

CHANGELOG (Unreleased), known-issues (the browser can open URLs; the
limits of openUrl: round trips per wave of misses, a call holds what it
reads, CORS and validator visibility, the download fallback), the M4
status in docs/design/range-reads.md (why NeedBytes rather than a
Worker, why not clawhdf5-remote's BlockCache, what was tested; the
status paragraph at the top lost a garbled duplicate), the viewer's
README (API, options, how it works, tests; the size table is marked as
predating openUrl), CLAUDE.md and the README crate list.

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

docs/known-issues.md

@@ -844,9 +844,10 @@ cache, but:
   `read_*_zerocopy`) need the file in memory and answer
   `FormatError::ContiguousStorageRequired` otherwise; `File::as_bytes()`
   panics for such a file (`File::contiguous_bytes()` is the fallible form).
-  `LazyFile`, `MmapFile` and the Python and wasm bindings still read a
-  whole file (`h5rs` reads through `File::storage`, and takes URLs with its
-  `remote` feature).
+  `LazyFile`, `MmapFile` and the Python bindings still read a whole file
+  (`h5rs` reads through `File::storage`, and takes URLs with its `remote`
+  feature; the wasm reader's `openUrl` reads by range requests since
+  2026-09-27, its `open(bytes)` takes a whole file).
 - The file's length is read once, at open: a growing file (SWMR) is not
   followed (milestone M5). A remote file is pinned at open, so one that
   grows is `RemoteError::FileChanged`.
@@ -862,9 +863,9 @@ cache, but:
 **Status:** open (added 2026-09-26, milestone M3 of
 `docs/design/range-reads.md`).
 
-- **Python and the browser cannot open URLs yet.** `clawhdf5.File` (PyO3)
-  parses through `File::as_bytes`, which a remote file does not have; the
-  wasm reader's `openUrl` is milestone M4.
+- **Python cannot open URLs yet.** `clawhdf5.File` (PyO3) parses through
+  `File::as_bytes`, which a remote file does not have. (The browser can
+  since 2026-09-27: `clawhdf5-wasm`'s `openUrl`, below.)
 - **The block size is fixed** (1 MiB unless `CacheConfig` says otherwise).
   The design's policy of using a paged file's page size as the block size
   is not implemented, and only the first block is read ahead.
@@ -903,10 +904,37 @@ cache, but:
 
 ## `clawhdf5-wasm` (browser) limits
 
-**Status:** open (by design for now; added 2026-09-26).
+**Status:** open (by design for now; added 2026-09-26, `openUrl` 2026-09-27).
 
-- The whole file is held in memory: `open()` takes its bytes. There are no
-  HTTP range reads, so a multi-GB file does not fit a browser tab.
+- `open()` holds the whole file in memory (it takes its bytes), so a
+  multi-GB local file does not fit a browser tab. A file on a web server
+  can be opened with `openUrl()` instead, which fetches only the byte
+  ranges each call needs (milestone M4 of `docs/design/range-reads.md`),
+  with these limits:
+  - **Round trips:** a call runs as passes over the blocks fetched so far
+    and is re-run after each wave of misses, so a call costs one round
+    trip per wave, not one for everything: a chunk index is walked a
+    level (or a node) per round trip, while the chunks of a read are
+    fetched together. Each pass re-parses what the call reads (CPU, not
+    network).
+  - **Memory:** a call keeps every block it reads until it finishes (the
+    cache budget applies between calls), so reading a large dataset whole
+    needs its stored bytes in memory next to the result; read windows
+    (`readHyperslab`) of large datasets. Files above 200 MB have not been
+    tested, nor offsets above 4 GiB on wasm32.
+  - **Cross-origin servers** must allow CORS for the page's origin and
+    either expose `Content-Range` (`Access-Control-Expose-Headers`) or
+    answer `HEAD` with `Content-Length`. The file is pinned at open by its
+    `ETag` or `Last-Modified` (and its length): when the page cannot see
+    either header, only a change of length is detected.
+  - **A server without range support** (it answers `200`) costs a whole
+    download, up to `maxDownload` (512 MiB), or an error with
+    `fallback: "error"`.
+  - Fixed block size (`blockSize`, 1 MiB by default); a paged file's page
+    size is not used. No retries: a failed request fails the call (calling
+    again retries it; what was fetched stays cached).
+  - Tested under Node 22 and headless Chromium (Playwright's build) against
+    a local server; not in Firefox or Safari.
 - Compound, reference, opaque, bitfield, time and VL-sequence datasets are
   refused with an error naming the type; attributes of those types come back
   as `value: null` with their `dtype`.