fix(format): locate the address in version-1 shared messages

A version-1 shared message reference is version, type, six reserved bytes
and then an old-style symbol table entry: link-name offset (length size),
object header address, cache type, reserved, scratch. We read the address
straight after the reserved bytes, i.e. the link-name offset, and the
committed datatype lookup failed with InvalidObjectHeaderVersion (the bytes
checked in tcompound.h5: name offset 0x10, then 0x590 = /type1). Datasets
of 1.4/1.6-era files that use a committed datatype were unreadable.

Skip the name offset. parse_shared_ref has no length size, so add
parse_shared_ref_sized and use it in every internal caller;
parse_shared_ref keeps its signature and assumes length size == offset
size. The old parse_v1_ref unit test encoded the wrong layout and now uses
the real bytes.

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

docs/known-issues.md

@@ -80,6 +80,8 @@ the VDS item, which is marked.
   - Hyperslab selection versions 1 and 2 are refused.
 - **Files with a user block:** the base address is not applied.
 - **Old-style shared messages (version 1)** read the wrong address.
+  **Fixed 2026-09-25:** the address follows the link-name offset of the
+  embedded symbol table entry.
 - **Groups and links:**
   - Groups with a user-defined link type (e.g. 187) cannot be listed.
   - Dense groups with more than about 22 000 links cannot be listed.