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

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=-
 ```