Don't let a missing serial listing discard an 'all' discovery on Windows

Windows CI (run 415) ran the discovery tests: 48 passed and one failed,
because port listing reads Linux sysfs and the test assumed it exists.
That is a real limitation, not just a bad test - and in the example it
meant `all` would listen and sweep and then abort on the serial step,
throwing the results away.

The serial step in `all` is now a printed note on failure; the standalone
`serial` command still errors. The Linux-only test is gated to Linux and
a counterpart asserts that off Linux the listing is an error, not a clean
empty result.

examples/discover_printers.rs

@@ -69,7 +69,12 @@ async fn main() -> anyhow::Result<()> {
                 // picking one would mean probing a network nobody named.
                 None => println!("(no subnet given, so no sweep - pass one like 192.168.0.0/24 to include it)"),
             }
-            found.extend(list_serial()?);
+            // Not `?`: serial listing reads Linux sysfs and fails on any other
+            // OS, and that must not throw away what the listen and sweep found.
+            match list_serial() {
+                Ok(ports) => found.extend(ports),
+                Err(err) => println!("(serial ports skipped: {err})"),
+            }
             found
         }
         "listen" | "raw" => {