fix(format): measure compound members by their stored size

The compound overlap check measured each earlier member with
Datatype::type_size, which is a fixed 16 for a variable-length type. On
disk a VL member takes 4 + offset size + 4 bytes, 12 in a file with
4-byte offsets, so a member right after one was refused as "member
overlaps with previous member" (and with the type, every attribute of
the object). libhdf5 measures members by their decoded, stored size
(times a v1 member's array dimensions); so does this now.

Reading VL values in such files is a separate, older gap, now recorded
in known-issues.

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

CHANGELOG.md

@@ -321,6 +321,9 @@
     half its bits unused (`Datatype::parse_in_header`,
     `Datatype::check_unused_bits`). A v1/v2 float's class bit 6 was read as
     VAX byte order; libhdf5 ignores it before version 3, and so does this.
+    The overlap check measures each earlier member by its stored size, as
+    libhdf5 does, so a variable-length member (4 + offset size + 4 bytes)
+    in a file with 4-byte offsets does not overlap the member after it.
   - chunked layouts (`FormatError::InvalidChunkDimensions`): a zero chunk
     dimension, a chunk rank that does not match the dataspace, a chunk of
     4 GiB or more indexed by a v1 B-tree (layout version 3 or earlier;