feat(client): report TAP IP in diagnostics

Add InterfaceUnicastAddress snapshots to client-route, backed by the Windows
unicast IP address table. The Windows client samples the TAP interface after it
resolves the adapter identity, preferring IPv4 for diagnostics and falling back
to the first address or unknown on lookup failure.

This keeps Win32 IP table handling in the route crate and fills the existing
TapDiagnostics IP field without making bridging depend on DHCP being present.
If DHCP has not assigned an address yet, diagnostics still make that visible as
unknown.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-client-route
- cargo test -p lanparty-client-win
- cargo check -p lanparty-client-route --target x86_64-pc-windows-gnu
- cargo check -p lanparty-client-route --target x86_64-pc-windows-gnu --tests
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check

Attempted:
- cargo check -p lanparty-client-route -p lanparty-client-win
  --target x86_64-pc-windows-gnu

Blocked because ring needs missing x86_64-w64-mingw32-gcc here.

Refs: PLAN.md

README.md

@@ -61,6 +61,7 @@ Windows route-table boundary:
 - scoped IP interface MTU overrides with restore-on-drop behavior
 - scoped IP interface metric overrides with restore-on-drop behavior
 - scoped default-route suppression with restore-on-drop behavior
+- unicast IP address snapshots for TAP diagnostics
 - scoped host-route pinning for the relay IP on the pre-TAP interface
 - non-Windows builds return a clear unsupported-platform error
 
@@ -156,4 +157,4 @@ rechecks that the relay route remains pinned, then restores the previous route
 policy on exit. Until automatic TAP MAC configuration is wired, startup fails
 before bridging if the driver-reported MAC does not match the tunnel identity.
 It prints client diagnostics snapshots with relay reachability, route-pinning,
-QUIC datagram budget, TAP status, frame/datagram counters, and drops.
+QUIC datagram budget, TAP status/IP, frame/datagram counters, and drops.