fix(tools): h5rs diff names its options as h5diff does

-c meant "list at most N differences" in h5rs, but in h5diff -c is
--compare (a flag) and the count is -n/--count=N, so a script moved over
from h5diff behaved differently: `h5diff -r -c 2 A B` exits 2 (the 2 is
taken as a file name) while h5rs exited 1.

The count is now -n/--count, -c/--compare is accepted (h5rs always lists
objects that are not comparable), and the --count=N, --delta=D,
--relative=R forms are accepted; exit codes equal h5diff's on 7 cases.

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

crates/clawhdf5-tools/src/diff.rs

@@ -29,7 +29,9 @@ soft link is compared as a link, as h5diff does, unless --follow-symlinks).
   -d, --delta D      numbers differ only when |a - b| > D
   -p, --relative R   numbers differ only when |a - b| / |a| > R (an R below
                      the f64 epsilon, 2.2e-16, compares exactly, as h5diff)
-  -c, --count N      list at most N differing elements per object with -r
+  -n, --count N      list at most N differing elements per object with -r
+  -c, --compare      list objects that are not comparable (always done;
+                     accepted for h5diff compatibility)
   --max-bytes N      largest dataset read (default 1 GiB); a larger one is an error
 
 Two NaNs compare equal. Unlike h5diff, objects that cannot be compared
@@ -139,7 +141,9 @@ pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
                 }
                 _ => return args.usage_error(out, "--relative needs a number >= 0", USAGE),
             },
-            "-c" | "--count" => match args.number() {
+            // h5diff's -c: h5rs always lists them.
+            "-c" | "--compare" => {}
+            "-n" | "--count" => match args.number() {
                 Some(n) => opts.count = n,
                 None => return args.usage_error(out, "--count needs a number", USAGE),
             },
@@ -151,6 +155,14 @@ pub fn run(args: &mut Args, out: &mut Out) -> std::io::Result<i32> {
                 writeln!(out.o, "{USAGE}")?;
                 return Ok(0);
             }
+            // h5diff's --count=N, --delta=D, --relative=R forms.
+            s if s.starts_with("--") && s.contains('=') => {
+                let (k, v) = s.split_once('=').unwrap_or((s, ""));
+                if !matches!(k, "--count" | "--delta" | "--relative" | "--max-bytes") {
+                    return args.usage_error(out, &format!("unknown option {a}"), USAGE);
+                }
+                args.push_front(k.to_string(), v.to_string());
+            }
             s if s.starts_with('-') && s.len() > 1 => {
                 return args.usage_error(out, &format!("unknown option {a}"), USAGE);
             }