fix(tools): h5rs diff compares soft links by target, like h5diff

An OBJ that was a soft link was resolved and its target object compared,
so two files whose /g/s both point at /z differed when /z did: exit 1,
where h5diff (without --follow-symlinks) compares the links' target paths
and exits 0.

A soft link is now compared as a link wherever it is, OBJ included.
--follow-symlinks compares the objects soft links lead to instead, walks
into soft-linked groups, resolves relative targets against the link's
group, and treats two dangling links as the same; exit codes equal
h5diff's on 14 cases. External links are never followed (documented).

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

crates/clawhdf5-tools/README.md

@@ -140,6 +140,7 @@ $ h5rs diff a.h5 b.h5 /x /y          # different paths in each
 $ h5rs diff -r a.h5 b.h5 /d          # list every differing element
 $ h5rs diff -d 0.001 a.h5 b.h5       # |a - b| > 0.001 is a difference
 $ h5rs diff -p 0.01 a.h5 b.h5        # |a - b| / |a| > 1% is a difference
+$ h5rs diff --follow-symlinks a.h5 b.h5 /lnk   # what the soft link /lnk leads to
 ```
 
 Integers are compared in integer arithmetic, with or without a tolerance, so
@@ -150,7 +151,14 @@ exactly, as h5diff's does.
 Exit status: 0 no differences, 1 differences, 2 error — the same as h5diff's
 on the cases `diff_exit_codes_match_h5diff` runs. Compared: which objects
 exist, their kinds, datatypes and shapes, attribute sets and values, dataset
-values, and soft/external link targets. Every path is compared: an object
+values, and soft/external link targets. A soft link — including an OBJ
+that is itself a soft link — is compared as a link, by its target path, as
+h5diff does; `--follow-symlinks` compares the objects soft links lead to
+instead (and walks into soft-linked groups), and two dangling links are then
+the same, as in h5diff. External links are always compared by target (file
+and path): `--follow-symlinks` does not open other files, where h5diff's
+does. Two dangling soft links with different targets are a difference
+(h5diff reports them with `-r`/`-v` but exits 0 without). Every path is compared: an object
 hard-linked under two names is compared under both, with everything below
 it, so a file that shares one object between two names equals a file that
 stores two identical copies. Differences from h5diff, on purpose: objects