fix(format): resolve VL elements as libhdf5 does

Checked with h5py on a patched file:
- a VL string with an embedded NUL reads up to the NUL (libhdf5 converts
  VL strings to C strings); read_vl_strings returned "a\0b";
- an element whose global heap object is not length x base size bytes is
  an error ("Expected global heap object size does not match"); we
  returned the object cut to the length;
- a heap address of 0 is a null element whatever its length.

vl_data::VlResolver does this, caching each parsed heap collection:
read_vl_strings parsed the whole collection again for every element.
read_vl_strings and read_vl_bytes use it; check_element_size refuses a VL
type whose stored element size is not 4 + offset size + 4. The
conformance probe resolves VL values through VlResolver instead of its
own lenient copy (575 of 697, unchanged).

The new unit tests fail against the old read_vl_strings.

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

CHANGELOG.md

@@ -16,6 +16,20 @@
   exhaustively destructures that variant; patterns with `..` are
   unaffected), and it is written back as stored. Tested against h5py
   (`crates/clawhdf5/tests/vl_offset4_interop.rs`).
+- **Wrong data: VL strings with an embedded NUL, and VL elements whose heap
+  object has the wrong size.** libhdf5 hands VL strings over as C strings,
+  so h5py reads `"a\0b"` as `"a"`; `read_vl_strings` returned the NUL and
+  what followed. An element whose heap object is not exactly
+  `length × base size` bytes is refused by libhdf5 ("Expected global heap
+  object size does not match"); we returned the object cut or padded to
+  the length. Both now behave as libhdf5, and a heap address of 0 is a null
+  element (empty) whatever its length. The new
+  `clawhdf5_format::vl_data::VlResolver` does this and parses each global
+  heap collection once per read: `read_vl_strings` parsed the whole
+  collection again for every element. `vl_data::check_element_size` refuses
+  a VL datatype whose stored size is not 4 + offset size + 4 (libhdf5
+  ignores the stored size). The conformance probe resolves VL elements
+  with `VlResolver` too; conformance unchanged at 575 of 697.
 
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