feat(obs): distinguish one-way broadcast flow

The client previously reported "Broadcast traffic flowing" as soon as either
broadcast TX or RX was nonzero. During the MVP DHCP/ARP proof, one-way
broadcast is useful but weaker evidence than bidirectional broadcast.

Keep the existing healthy message for two-way broadcast, but report
outbound-only broadcast as a warning that the client is still waiting for a LAN
broadcast reply, and report inbound-only broadcast separately. This makes the
Windows client status lines more precise when DHCP is stuck.

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

Refs: MVP Windows DHCP diagnostics

README.md

@@ -271,7 +271,8 @@ after bridging starts become visible in later status lines, preferring a
 non-link-local IPv4 address when Windows reports several TAP addresses. Each
 snapshot also emits short user-facing lines such as relay/gateway connection status,
 relay-route and TAP readiness warnings, DHCP address presence, relay RTT, and
-broadcast-flow confirmation when those signals are observed. Malformed frames
+broadcast-flow confirmation. One-way broadcast diagnostics distinguish frames
+sent toward the LAN from broadcast frames received back from the LAN. Malformed frames
 read from TAP, invalid or unauthorized source-MAC frames, L2 control-plane
 traffic, remote VLAN tags, DHCP server replies, IPv6 Router Advertisements, IPv6
 fragments, jumbo frames, and TAP frames whose encoded datagrams exceed the

TESTING.md

@@ -246,6 +246,8 @@ Client health:
 ```text
 Relay RTT: 23 ms
 Broadcast traffic flowing
+Broadcast sent toward LAN; waiting for LAN broadcast reply
+LAN broadcast received
 client frame direction=TapToRelay ... action=Forwarded drop_reason=-
 client frame direction=RelayToTap ... action=Forwarded drop_reason=-
 ```

crates/lanparty-obs/src/lib.rs

@@ -447,11 +447,23 @@ impl ClientDiagnostics {
             ));
         }
 
-        if self.stats.broadcast_frames_tx() > 0 || self.stats.broadcast_frames_rx() > 0 {
+        match (
-            diagnostics.push(UserDiagnostic::new(
+            self.stats.broadcast_frames_tx() > 0,
+            self.stats.broadcast_frames_rx() > 0,
+        ) {
+            (true, true) => diagnostics.push(UserDiagnostic::new(
                 UserDiagnosticLevel::Info,
                 "Broadcast traffic flowing",
-            ));
+            )),
+            (true, false) => diagnostics.push(UserDiagnostic::new(
+                UserDiagnosticLevel::Warning,
+                "Broadcast sent toward LAN; waiting for LAN broadcast reply",
+            )),
+            (false, true) => diagnostics.push(UserDiagnostic::new(
+                UserDiagnosticLevel::Info,
+                "LAN broadcast received",
+            )),
+            (false, false) => {}
         }
 
         diagnostics
@@ -723,6 +735,50 @@ mod tests {
         assert_eq!(user_diagnostics[2].level(), UserDiagnosticLevel::Warning);
     }
 
+    #[test]
+    fn distinguishes_one_way_broadcast_diagnostics() {
+        let mut diagnostics = ClientDiagnostics::new(
+            RelayDiagnostics::new(true, true, true),
+            QuicDiagnostics::new(true, Some(1400)),
+            TapDiagnostics::new(
+                true,
+                Some(MacAddr::new([0x02, 1, 2, 3, 4, 5])),
+                Some(1200),
+                None,
+            ),
+            TunnelStats::default().with_broadcast_frames(1, 0),
+        );
+        let broadcast = diagnostics
+            .user_diagnostics()
+            .into_iter()
+            .find(|diagnostic| diagnostic.message().contains("Broadcast"))
+            .expect("outbound broadcast should produce a diagnostic");
+        assert_eq!(broadcast.level(), UserDiagnosticLevel::Warning);
+        assert_eq!(
+            broadcast.message(),
+            "Broadcast sent toward LAN; waiting for LAN broadcast reply"
+        );
+
+        diagnostics = ClientDiagnostics::new(
+            RelayDiagnostics::new(true, true, true),
+            QuicDiagnostics::new(true, Some(1400)),
+            TapDiagnostics::new(
+                true,
+                Some(MacAddr::new([0x02, 1, 2, 3, 4, 5])),
+                Some(1200),
+                None,
+            ),
+            TunnelStats::default().with_broadcast_frames(0, 1),
+        );
+        let broadcast = diagnostics
+            .user_diagnostics()
+            .into_iter()
+            .find(|diagnostic| diagnostic.message().contains("broadcast"))
+            .expect("inbound broadcast should produce a diagnostic");
+        assert_eq!(broadcast.level(), UserDiagnosticLevel::Info);
+        assert_eq!(broadcast.message(), "LAN broadcast received");
+    }
+
     #[test]
     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();