fix(tunnel): enforce negotiated TAP MTU

The MVP tunnel negotiates an effective TAP MTU and configures the Windows TAP IP
interface to that value, but the forwarding path only rejected frames that were
standard-Ethernet jumbo frames or exceeded the QUIC datagram budget. A frame
could therefore be larger than the negotiated TAP MTU while still fitting inside
the QUIC datagram budget.

Make the TAP-MTU frame limit an explicit shared protocol helper and enforce it
at every data-path boundary: Windows client send/receive, Linux gateway
send/receive, and relay forwarding. Such frames now produce TapMtuExceeded in
logs and counters instead of being forwarded until a later layer drops or
accepts them implicitly.

This keeps the no-fragmentation contract honest: one Ethernet frame still maps
to one QUIC datagram, but only if that frame also fits the room's negotiated TAP
MTU.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-proto tap_mtu
- cargo test -p lanparty-client-core connects_to_relay_control_stream_as_client
- cargo test -p lanparty-gateway connects_to_relay_control_stream_as_gateway
- cargo test -p lanparty-relay drops_frames_above_effective_tap_mtu
- cargo test -p lanparty-relay rate_limits_client_total_bandwidth_after_burst
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo build --release -p lanparty-relay -p lanparty-gateway
- git diff --check
- git diff --cached --check

Refs: MVP no-fragmentation tunnel MTU contract

TESTING.md

@@ -256,6 +256,7 @@ Drops that can be normal during testing:
 
 ```text
 drop_reason=UnknownDestination
+drop_reason=TapMtuExceeded
 drop_reason=DatagramBudget
 drop_reason=RateLimit
 ```
@@ -263,6 +264,9 @@ drop_reason=RateLimit
 On gateway `LanToRemote` logs, `UnknownDestination` usually means the gateway
 captured unrelated LAN unicast and dropped it locally instead of sending it to
 the relay.
+`TapMtuExceeded` means a host emitted an Ethernet frame larger than the
+negotiated tunnel MTU; occasional drops can happen while testing software that
+does not honor the smaller adapter MTU yet.
 
 Drops that should be investigated if they dominate: