fix(obs): warn on TAP link-local IPv4

IPv4 link-local on the TAP adapter means Windows self-assigned APIPA because
LAN DHCP did not complete through the tunnel. The previous user diagnostic
only said "TAP IP detected", which made a failed DHCP path look neutral
during MVP testing.

Return a full UserDiagnostic from TAP IP classification so IPv4 link-local
can be a warning while normal IPv4 still reports the received DHCP address.
Keep IPv6 link-local neutral because it is expected on many Windows
interfaces and is not evidence of LAN DHCP success or failure.

TESTING.md now tells the operator to troubleshoot 169.254.x.x like
`Waiting for TAP IP`.

Test Plan:
- cargo test -p lanparty-obs
- cargo test -p lanparty-client-win
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo fmt --check
- git diff --check

Refs: PLAN.md diagnostics; TESTING.md MVP guide

TESTING.md

@@ -242,6 +242,10 @@ If the client says `Waiting for TAP IP`, DHCP is not making the full round trip.
 Check relay/gateway frame logs for broadcast traffic and check that the gateway
 is on wired Ethernet.
 
+If the client reports a TAP link-local address such as `169.254.x.x`, treat it
+the same way: Windows has self-assigned an address, but LAN DHCP did not
+complete through the tunnel.
+
 If startup fails with a TAP MAC mismatch, disable/enable the TAP adapter or
 reinstall TAP-Windows6 so Windows reloads the `NetworkAddress` value. Do not
 continue with a mismatched MAC.

crates/lanparty-obs/src/lib.rs

@@ -434,8 +434,8 @@ impl ClientDiagnostics {
             ));
         }
 
-        if let Some(message) = tap_ip_user_message(self.tap.ip()) {
-            diagnostics.push(UserDiagnostic::new(UserDiagnosticLevel::Info, message));
+        if let Some(diagnostic) = tap_ip_user_diagnostic(self.tap.ip()) {
+            diagnostics.push(diagnostic);
         }
 
         if let Some(rtt_ms) = self.quic.relay_rtt_ms() {
@@ -456,10 +456,20 @@ impl ClientDiagnostics {
     }
 }
 
-fn tap_ip_user_message(ip: Option<IpAddr>) -> Option<String> {
+fn tap_ip_user_diagnostic(ip: Option<IpAddr>) -> Option<UserDiagnostic> {
     match ip {
-        Some(IpAddr::V4(ip)) if !ip.is_link_local() => Some(format!("DHCP received: {ip}")),
-        Some(ip) => Some(format!("TAP IP detected: {ip}")),
+        Some(IpAddr::V4(ip)) if ip.is_link_local() => Some(UserDiagnostic::new(
+            UserDiagnosticLevel::Warning,
+            format!("TAP has link-local IP {ip}; waiting for LAN DHCP"),
+        )),
+        Some(IpAddr::V4(ip)) => Some(UserDiagnostic::new(
+            UserDiagnosticLevel::Info,
+            format!("DHCP received: {ip}"),
+        )),
+        Some(ip) => Some(UserDiagnostic::new(
+            UserDiagnosticLevel::Info,
+            format!("TAP IP detected: {ip}"),
+        )),
         None => None,
     }
 }
@@ -712,14 +722,21 @@ mod tests {
     }
 
     #[test]
-    fn avoids_calling_link_local_tap_ip_dhcp() {
+    fn reports_link_local_tap_ipv4_as_waiting_for_lan_dhcp() {
+        let diagnostic = tap_ip_user_diagnostic(Some("169.254.10.20".parse().unwrap())).unwrap();
+
+        assert_eq!(diagnostic.level(), UserDiagnosticLevel::Warning);
         assert_eq!(
-            tap_ip_user_message(Some("169.254.10.20".parse().unwrap())),
-            Some("TAP IP detected: 169.254.10.20".to_string())
-        );
-        assert_eq!(
-            tap_ip_user_message(Some("fe80::1".parse().unwrap())),
-            Some("TAP IP detected: fe80::1".to_string())
+            diagnostic.message(),
+            "TAP has link-local IP 169.254.10.20; waiting for LAN DHCP"
         );
     }
+
+    #[test]
+    fn reports_ipv6_link_local_tap_ip_without_calling_it_dhcp() {
+        let diagnostic = tap_ip_user_diagnostic(Some("fe80::1".parse().unwrap())).unwrap();
+
+        assert_eq!(diagnostic.level(), UserDiagnosticLevel::Info);
+        assert_eq!(diagnostic.message(), "TAP IP detected: fe80::1");
+    }
 }