docs: changelog and known issues for the format robustness work

Co-Authored-By: Claude Fable 5.1 <[email protected]>

docs/known-issues.md

@@ -107,3 +107,27 @@ only attributes convertible to `AttrValue`. An attribute with, e.g., a compound
 datatype is omitted from the map with no error or indication that it exists.
 Planned: surface these as an explicit `AttrValue` variant (raw bytes + datatype)
 or an error, as part of the "no silent skips" robustness work.
+
+## B-tree v2 chunk index (layout v4, index type 5) is not supported
+
+**Status:** open.
+
+**Summary:** a chunked dataset with **two or more unlimited dimensions** written
+with `libver='latest'` indexes its chunks with a version-2 B-tree. Reading it
+fails with `ChunkedReadError("unsupported chunked layout version=4,
+index_type=Some(5)")`. Single-chunk, implicit, fixed-array and
+extensible-array indexes (and the v3 B-tree v1) are supported.
+
+**Repro:** `f.create_dataset("d", shape=(5, 7), chunks=(2, 3), maxshape=(None, None))`
+with `h5py.File(..., libver='latest')`.
+
+## External links and external raw data are not followed
+
+**Status:** open (by design for now); both are explicit errors.
+
+**Summary:** a path through an external link returns
+`FormatError::ExternalLinkUnsupported { filename, object_path }`, and a dataset
+created with `external=[...]` storage returns
+`FormatError::ExternalDataFilesUnsupported`. Neither is resolved. If support is
+added, file names must be confined to the opened file's directory, as the
+virtual-dataset resolver now does.