feat(facade): read VL strings and VL sequences through File

VL-string datasets (h5py's default str dtype) failed read_string with
"type mismatch: expected String, got VariableLength". read_string now
reads fixed- and variable-length strings, with h5py's values (a string
ends at a NUL, a null element is ""). New:
- Dataset::read_string_bytes: each VL string's exact bytes;
- Dataset::read_string_selection: hyperslabs/points of either kind;
- Dataset::read_vlen::<T>() and read_vlen_selection::<T>(): VL sequences
  of numbers as Vec<Vec<T>>, T in f64/f32/i64/i32/u64, converted like the
  other typed readers;
- File::decode_strings / decode_string_bytes / decode_vlen: VL values in
  compound fields and AttrValue::Raw attributes;
- MmapDataset and LazyDataset: read_string for VL strings,
  read_string_bytes and read_vlen.

tests/vl_data_interop.rs checks every path against h5py with 8- and
4-byte offsets: scalar, 1-D and 2-D, ASCII and UTF-8, empty strings,
contiguous, compact, chunked with gzip and shuffle, unwritten and partly
written chunks, hyperslabs, compound members, attributes, a big-endian
base type, and a patched file with an embedded NUL and mis-sized heap
objects. NetCDF-4 string variables read too (netCDF4-python test).

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

docs/known-issues.md

@@ -140,7 +140,13 @@ fill-value item that did is fixed).
   is left out of `attrs()` (reported by `attrs_with_errors()`) instead of
   failing the others.
 - **Other readers:**
-  - VL-string datasets are not readable through `File`.
+  - VL-string datasets are not readable through `File`. **Fixed
+    2026-09-26:** `read_string` reads them (also `read_string_bytes`,
+    `read_string_selection`, and on `MmapFile`/`LazyFile`), with h5py's
+    values: strings end at a NUL, null elements are `""`; VL sequences of
+    numbers read with `read_vlen::<T>()`, and VL values inside compounds or
+    `AttrValue::Raw` attributes decode with `File::decode_strings` /
+    `File::decode_vlen` (`crates/clawhdf5/tests/vl_data_interop.rs`).
   - Variable-length values inside a compound (and VL-string attributes) in
     a file with 4-byte offsets (`sizeof_addr = 4`) fail with
     `GlobalHeapObjectNotFound` or come back as `Raw`: these paths assume
@@ -505,7 +511,8 @@ which is what libhdf5 itself writes.
   followed (no file system).
 - Variable-length string datasets are read by decoding `read_selection`'s
   bytes with `clawhdf5_format::vl_data` in the wasm crate; `File` itself still
-  cannot (see the audit gaps above).
+  cannot (see the audit gaps above). (`File` can since 2026-09-26; the wasm
+  crate still decodes them itself.)
 
 ## The Node.js package (`packages/clawhdf5-node`) does not work