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" => {

src/discovery/serial.rs

@@ -485,6 +485,7 @@ mod tests {
 
     /// The one part of serial discovery this environment can demonstrate:
     /// with no ports attached it finds none, and does not error doing it.
+    #[cfg(target_os = "linux")]
     #[test]
     fn listing_ports_where_there_are_none_is_empty_and_not_an_error() {
         let ports = list_ports().expect("sysfs exists on this host");
@@ -492,4 +493,14 @@ mod tests {
             assert!(port.path.starts_with("/dev/"), "{port:?}");
         }
     }
+
+    /// Port listing reads Linux sysfs, so elsewhere it must say so rather
+    /// than report "no ports" - which would look like a clean scan (found on
+    /// Windows CI, run 415, where the Linux test above failed).
+    #[cfg(not(target_os = "linux"))]
+    #[test]
+    fn listing_ports_off_linux_is_an_error_not_an_empty_list() {
+        let err = list_ports().expect_err("there is no sysfs here");
+        assert!(err.to_string().contains("not a Linux host"), "{err}");
+    }
 }