fix(format): read huge, tiny and filtered fractal heap objects

A heap ID's type is in bits 4-5 of its first byte (H5HF_ID_TYPE_MASK
0x30); bits 6-7 are the ID version. The reader took the type from bits
6-7, so every huge object ID (0x10) was decoded as a managed one and
failed — and since dense attributes are read all at once, one attribute
over the heap's 4 KiB managed limit made every attribute on its object
unreadable (netcdf4-python's issue671.nc / issue672.nc).

- Huge objects (type 1): located directly from the ID when address and
  length fit in it, otherwise through the huge-object v2 B-tree (record
  types 1 and 2); filtered huge objects are decoded with the heap's
  pipeline and their filter mask.
- Tiny objects (type 2): read from the ID itself.
- Filtered heaps: the header's pipeline is parsed (it was skipped short,
  so the header checksum was read from the wrong place), indirect-block
  entries for direct blocks carry their filtered size and mask, and
  direct blocks are decoded before objects are read from them.
- An unknown ID version is an error.

FractalHeapHeader gains huge_btree_address, filter_pipeline,
root_direct_block_filtered_size, root_direct_block_filter_mask,
offset_size and length_size; read_managed_object now accepts any ID type.

Regression tests (h5py-written, compared with h5py):
dense_attribute_stored_as_a_huge_heap_object, dense_group_with_a_huge_link,
dense_group_with_a_filtered_link_heap; unit tests
tiny_object_is_read_from_the_id, huge_object_with_a_direct_id,
unknown_heap_id_version_is_refused.

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

docs/known-issues.md

@@ -83,7 +83,8 @@ the VDS item, which is marked.
   - Soft links are left out of `datasets()`.
 - **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`).
+  files (`issue671.nc`). **Fixed 2026-09-25:** huge and tiny heap objects,
+  and filtered heaps, are read.
 - **Other readers:**
   - VL-string datasets are not readable through `File`.
   - Metadata cache images are not supported.