feat(client): add relay session core

lanparty-client-core now owns the platform-neutral remote client relay session.
It validates relay trust input, room, virtual MAC identity, and datagram budget,
then connects to the relay, sends a client hello, and stores the assigned
welcome metadata.

The session exposes Ethernet datagram send and receive helpers that stamp
outgoing frames with the relay-assigned room and peer ids, ignore frames for
other rooms or from itself, and reject malformed Ethernet payloads before
handing them to the future TAP bridge.

The loopback Quinn test verifies the full client-side control and datagram path
without requiring Windows TAP access: pinned certificate trust, role = client
hello, announced virtual MAC, welcome parsing, and Ethernet datagrams in both
directions.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings

Refs: PLAN.md Windows client relay connection and QUIC datagrams

README.md

@@ -38,6 +38,15 @@ Shared diagnostics and structured logging vocabulary:
 - tunnel counters shared by control messages and runtime diagnostics
 - client connectivity/TAP diagnostics and user-facing status messages
 
+### `lanparty-client-core`
+
+Platform-neutral remote client relay session:
+
+- relay QUIC connection with pinned relay certificate trust
+- client hello with room, virtual MAC, and datagram budget
+- welcome/reject handling with assigned peer id and effective TAP MTU
+- Ethernet frame send/receive helpers over QUIC DATAGRAM
+
 ### `lanparty-relay`
 
 Public relay binary and relay-owned room state: