feat(client): print runtime diagnostics snapshots

The client now builds ClientDiagnostics snapshots from the connected session,
known TAP state, route-pinning status, and tunnel counters. Windows prints one
snapshot after TAP and relay-route setup, then repeats snapshots while bridging
so frame/datagram counters and drops are visible during manual phase-1 tests.

Non-Windows builds print the same relay and QUIC diagnostics with TAP fields
marked unknown before waiting for Ctrl-C. TAP IP remains unknown until a later
Windows adapter IP inspection slice wires that source of truth.

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

Refs: PLAN.md

README.md

@@ -155,3 +155,5 @@ interface metric and disables TAP default routes while it runs, periodically
 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.