feat(wasm): examples/wasm-viewer, an HDF5/NetCDF-4 viewer page

Drop a file (or pass ?file=<url>&path=<object>), browse the tree lazily,
see a dataset's type, shape, max shape and attributes, and page through
its values as 50x12 hyperslab windows (leading dims of 3-D+ data held
at chosen indices). build.sh produces pkg/ (not committed) with
wasm-bindgen --target web and checks the CLI matches the crate version.

test/run.sh builds it and runs test.mjs under Node against the h5py/
netCDF4 fixture (250 checks: every dataset whole and as a strided
hyperslab, listings, attributes, error paths, the page's DOM-free
helpers), then browser.sh renders the page in headless Chromium for
eight objects and checks the DOM. The fixture gains LZ4 (read) and Zstd
(refused: links C) datasets and a compound attribute (value null plus
its type). ci-test.sh runs it when node and wasm-bindgen exist; the CI
container has neither, so CI relies on the native h5py_interop test.

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

examples/wasm-viewer/README.md

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+# HDF5 viewer in the browser
+
+A single page that opens an HDF5 or NetCDF-4 file entirely in the browser
+with `clawhdf5-wasm` (clawhdf5's reader compiled to WebAssembly): drop a
+file, browse its groups, and look at a dataset's type, shape, attributes
+and values (a 50 x 12 window at a time, read as a hyperslab, with the
+leading dimensions of a 3-D+ dataset held at chosen indices). The file never
+leaves the page.
+
+## Build and open
+
+```bash
+rustup target add wasm32-unknown-unknown
+cargo install wasm-bindgen-cli --version 0.2.129   # must equal the crate version; build.sh checks
+bash examples/wasm-viewer/build.sh                  # writes examples/wasm-viewer/pkg/ (not committed)
+python3 -m http.server -d examples/wasm-viewer 8000 # wasm cannot load from file://
+```
+
+Then open <http://localhost:8000/>. `?file=<url>&path=<object>` opens a
+file from a URL (same origin, or one serving CORS headers) and selects an
+object in it, e.g. `?file=data/run1.h5&path=/results/energy`.
+
+## JavaScript API
+
+```js
+import init, { open } from "./pkg/clawhdf5_wasm.js";
+await init();
+const f = open(new Uint8Array(await blob.arrayBuffer()));
+f.list("/");                          // [{ name, kind: "group" | "dataset" }], groups first
+f.info("/grid");                      // { shape, maxshape, dtype, elementShape }
+f.attrs("/grid");                     // [{ name, value, dtype }]
+f.read("/grid");                      // { shape, dtype, data }
+f.readHyperslab("/grid", [0, 0], [10, 5], [2, 1]);  // start, count, stride?, block?
+f.free();
+```
+
+`data` is the typed array of the stored width (`Float64Array`,
+`Float32Array` also for `f16`, `Int8Array` ... `BigInt64Array`,
+`BigUint64Array`), or an array of strings for fixed- and variable-length
+strings and enumerations (h5py booleans read as `"TRUE"`/`"FALSE"`). Array
+datatypes are flattened, their dimensions appended to `shape`. Anything
+else throws an `Error` naming the type.
+
+## Limits
+
+- Read-only, and the whole file is held in memory (no range requests).
+- Compound, reference, opaque and variable-length-sequence datasets are
+  refused with an error. Attributes of those types are listed with
+  `value: null` and their `dtype`.
+- No Zstd or SZIP filters (they link C): such a dataset fails with
+  `unsupported filter`. Deflate, shuffle, Fletcher-32, LZ4, N-Bit and
+  scale-offset are read (within the limits in `docs/known-issues.md`).
+- Virtual datasets whose sources are in other files, and external links,
+  cannot be followed: there is no file system.
+
+## Tests
+
+`test/run.sh` builds the package, writes `fixture.h5` (h5py) and
+`fixture.nc` (netCDF4) with `test/make_fixture.py`, then:
+
+- runs `test/test.mjs` under Node: every dataset (whole and a strided
+  hyperslab), listing and attribute is compared with what libhdf5 reads
+  back, error paths are checked, and so are the page's DOM-free helpers
+  (`viewer-lib.js`);
+- runs `test/browser.sh`: loads the page in headless Chromium with
+  `?file=fixture.h5&path=...` for eight objects and checks the rendered tree,
+  types, shapes, attribute and value cells, and the error shown for an
+  unsupported type. Skipped when no Chromium is found (`CHROME` names one;
+  a Playwright download under `~/.cache/ms-playwright` is picked up).
+  Drag-and-drop and the file picker are not driven by it; they share
+  `load()` with the `?file=` path.
+
+The same expectations are checked natively, without Node, by
+`crates/clawhdf5-wasm/tests/h5py_interop.rs`, which is what CI runs (the CI
+container has no Node or browser).
+
+## Size
+
+Measured 2026-09-26 on tank (rustc 1.98.1, wasm-bindgen 0.2.129, gzip 1.14,
+`gzip -9 -n`), after `bash examples/wasm-viewer/build.sh`:
+
+| | raw | gzip -9 |
+|---|---:|---:|
+| `pkg/clawhdf5_wasm_bg.wasm` (profile `wasm-release`, opt-level `s`) | 627,501 B | 191,639 B |
+| `pkg/clawhdf5_wasm.js` (wasm-bindgen glue) | 21,826 B | 4,487 B |
+| same wasm at opt-level `z` | 693,068 B | 192,550 B |
+| same wasm at opt-level `3` | 544,035 B | 198,803 B |
+| h5wasm 0.10.3: wasm embedded in `dist/esm/hdf5_util.js` | 3,544,184 B | 907,096 B |
+| h5wasm 0.10.3: `dist/esm/hdf5_util.js` as shipped | 4,150,134 B | 986,699 B |
+
+h5wasm figures: `npm pack [email protected]` (npm reports
+`dist.unpackedSize` 14,731,385 B for the whole package), wasm extracted from
+the `binaryDecode` literal in `hdf5_util.js`. h5wasm is the whole of libhdf5
+(writing, every datatype, plugins), so this compares download size, not
+equal functionality. No `wasm-opt` pass was applied (binaryen is not
+installed on tank). opt-level `s` is used because it is the smallest
+compressed.