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]>

docs/known-issues.md

@@ -129,6 +129,12 @@ fill-value item that did is fixed).
   failing the others.
 - **Other readers:**
   - VL-string datasets are not readable through `File`.
+  - 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
+    the 16-byte element of an 8-byte-offset file. The datatype itself reads
+    (it was refused as "member overlaps with previous member" until
+    2026-09-26).
   - Metadata cache images are not supported.
   - x87 long double and binary128 are refused.
   - N-Bit on 64-bit scale-offset data and some N-Bit parameter layouts fail.