feat: M3 seed — the lumen/1 native protocol: QUIC control plane + reference client (Phase 5)

The first end-to-end run of lumen's own protocol, past the GameStream compatibility layer.

- lumen-core/src/quic.rs (behind the `quic` feature): the lumen/1 handshake — Hello/Welcome/
  Start as length-prefixed LE binary on one QUIC bi-stream. Welcome carries the COMPLETE
  data-plane Config: mode, FEC scheme incl. GF(2^16) Leopard (inexpressible in GameStream),
  shard sizing, AES-GCM key + per-direction salt, data UDP port. Plus quinn endpoint helpers
  (self-signed server; accepts-any client — pinning lands with the trust model) and framed
  async IO. Round-trip unit-tested.
- lumen-host m3-host: serves one lumen/1 session — QUIC handshake, then a NATIVE thread
  (no async on the frame path — design invariant) streams deterministic 64KB test frames
  through the hardened M1 Session over UdpTransport.
- lumen-client-rs: from scaffold to working reference client — connects, negotiates, brings
  up the client Session over UDP, reassembles + FEC-recovers + byte-verifies every frame.

VALIDATED END-TO-END on localhost: 300/300 frames verified, 0 mismatches, through
QUIC-negotiated GF(2^16) FEC + AES-GCM over real UDP sockets. M4 (decode+present) builds on
this exact client skeleton.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/lumen-host/Cargo.toml

@@ -9,7 +9,9 @@ authors.workspace = true
 repository.workspace = true
 
 [dependencies]
-lumen-core = { path = "../lumen-core" }
+lumen-core = { path = "../lumen-core", features = ["quic"] }
+# M3 native control plane (the `lumen/1` QUIC handshake; data plane stays native-thread UDP).
+quinn = "0.11"
 anyhow = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }