feat(client): log TAP frame movement

The relay and gateway already emit structured frame logs, but the Windows
client only exposed aggregate counters. During the MVP end-to-end test that
left a blind spot between TAP reads/writes and the relay datagram path.

Add client-side frame log lines for accepted TAP-to-relay sends,
relay-to-TAP writes, and local TAP-frame drops before relay send. The logs use
the shared FrameLog vocabulary with TapToRelay and RelayToTap directions so the
client, relay, and gateway logs can be correlated during DHCP, ARP, ping, and
LAN-game discovery checks.

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

Refs: MVP Windows client diagnostics

crates/lanparty-obs/src/lib.rs

@@ -10,6 +10,8 @@ use lanparty_proto::{EthernetFrame, EthernetSafetyDrop, MacAddr};
 #[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Deserialize, serde::Serialize)]
 #[serde(rename_all = "snake_case")]
 pub enum FrameDirection {
+    TapToRelay,
+    RelayToTap,
     RemoteToLan,
     LanToRemote,
     RelayIngress,