fix(format): list soft links as their targets, like h5py

Group::datasets()/groups() (and the Mmap/Lazy handles) listed only hard
links, so a soft link to a dataset or group was missing, and dataset(name)
/ group(name) on a group handle could not open one. In old-style (symbol
table) groups a soft link's entry has no object header address, and the
listing failed outright trying to parse one.

The three facade handles each had their own copy of the child-listing
code; they now share group_v2::resolve_group_children, which returns hard
links plus soft links resolved to their targets (relative targets from
the group holding the link, via the new resolve_path_from). A dangling or
cyclic soft link, an external link and a user-defined link are left out —
h5py lists their names but cannot open them. Any other error met while
resolving is returned, not hidden.

Path resolution now walks a relative soft link's target from the group
holding it instead of rebuilding the path from the root (same result,
one less re-walk), and ignores "." components.

Regression test: soft_links_are_listed_as_their_targets (h5py writes
absolute, relative, group, dangling, cyclic and external links with
libver latest and earliest; listings compared with h5py for File,
MmapFile and LazyFile).

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

docs/known-issues.md

@@ -82,7 +82,8 @@ the VDS item, which is marked.
     **Fixed 2026-09-25:** two bugs — fractal-heap child indirect blocks had
     the wrong row count, and v2 B-tree internal nodes at depth 3+ were read
     with the wrong pointer widths.
-  - Soft links are left out of `datasets()`.
+  - Soft links are left out of `datasets()`. **Fixed 2026-09-25:** soft
+    links are listed as their targets; dangling ones are left out.
 - **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`). **Fixed 2026-09-25:** huge and tiny heap objects,