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]>

CHANGELOG.md

@@ -128,15 +128,17 @@
   - `h5rs stat FILE` reports h5stat's object, link, rank, layout, filter,
     attribute, raw-data and file-size figures (equal to h5stat's on the test
     files); metadata space is one figure, not broken down.
-  - `h5rs diff [-r] [-q] [-d D] [-p R] A B [OBJ1 [OBJ2]]` compares objects,
-    kinds, datatypes, shapes, attributes, values and link targets; exit
-    status 0/1/2 as h5diff's. Every path is compared, including every name
-    of a hard-linked object and the members of a hard-linked group; with a
-    `-d`/`-p` tolerance, integers are compared exactly in integer
-    arithmetic (no loss above 2^53), and a `-p` below the f64 epsilon
-    compares exactly, as h5diff's. Objects
-    that cannot be compared count as a difference (h5diff exits 0 for them),
-    and NaN equals NaN.
+  - `h5rs diff [-r] [-q] [-d D] [-p R] [--follow-symlinks] A B [OBJ1 [OBJ2]]`
+    compares objects, kinds, datatypes, shapes, attributes, values and link
+    targets; exit status 0/1/2 as h5diff's. Soft links are compared by
+    target path, as h5diff's default, or with `--follow-symlinks` by the
+    objects they lead to (external links are never followed). Every path is
+    compared, including every name of a hard-linked object and the members
+    of a hard-linked group; with a `-d`/`-p` tolerance, integers are
+    compared exactly in integer arithmetic (no loss above 2^53), and a `-p`
+    below the f64 epsilon compares exactly, as h5diff's. Objects that cannot
+    be compared count as a difference (h5diff exits 0 for them), and NaN
+    equals NaN.
   - `h5rs check [--data] FILE` is a structural validator: it walks every
     object, parses every header message, verifies the checksums of every
     version 2+ structure it meets (superblock, object headers and