Merge branch 'fix/p1-userblock-shared' into fix/p1-read-gaps

# Conflicts:
#	crates/clawhdf5-format/src/attribute.rs
#	crates/clawhdf5-format/src/datatype.rs
#	crates/clawhdf5-format/src/shared_message.rs
#	docs/known-issues.md

docs/known-issues.md

@@ -89,13 +89,23 @@ the VDS item, which is marked.
   - `%b` printf-style source names are not expanded.
   - Hyperslab selection versions 1 and 2 are refused.
 - **Files with a user block:** the base address is not applied.
+  **Fixed 2026-09-25:** every reader views the file from the superblock on
+  (`twithub.h5`, `twithub513.h5`, `h5clear_fsm_persist_user_*.h5`; the
+  `twithub` files still stop at the user-defined link type below).
 - **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.
+  **Fixed 2026-09-25:** the address follows the length-sized link-name
+  offset of the embedded symbol table entry (`tcompound.h5`, `tcompound2.h5`).
+- **Array members of version-1 compound datatypes** (found while fixing the
+  items above) were read as one element — wrong data. **Fixed 2026-09-25.**
 - **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.
   - Soft links are left out of `datasets()`.
+  - **Wrong data (found while fixing user blocks):** an old-style group whose
+    local-heap free list points outside the heap listed garbage names where
+    libhdf5 refuses the heap. **Fixed 2026-09-25**
+    (`InvalidLocalHeapFreeList`, checked when a name is first read, as
+    libhdf5 does).
 - **Dense attributes:** a large attribute stored as a fractal-heap "huge"
   object makes every attribute on the object fail. This affects real NetCDF
   files (`issue671.nc`).