fix(gateway): capture whole LAN frames before budget checks

The gateway AF_PACKET read path used the standard 1514 byte Ethernet frame
length as its receive buffer. VLAN-tagged or jumbo LAN frames could therefore
be truncated before the bridge reached the encoded-datagram budget check, so
logs and drop accounting saw a corrupted shorter frame.

Use an overlay payload-sized capture buffer instead. This lets the Linux
gateway observe the whole frame that the kernel reports, then leave the
existing Ethernet parsing and negotiated QUIC datagram budget checks to decide
whether the frame can cross the tunnel. The bridge still never fragments
Ethernet frames.

Document the behavior in the gateway README section and add a compile-time
guard so the capture buffer stays above the standard Ethernet frame size.

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

Refs: PLAN.md

crates/lanparty-gateway/src/lib.rs

@@ -32,8 +32,7 @@ use lanparty_obs::{DropReason, TunnelStats};
 #[cfg(target_os = "linux")]
 use lanparty_obs::{FrameAction, FrameDirection, FrameLog};
 use lanparty_proto::{
-    EthernetFrame, FrameType, MAX_STANDARD_ETHERNET_FRAME_LEN, MacAddr, decode_datagram,
-    encode_datagram, validate_datagram_budget,
+    EthernetFrame, FrameType, MacAddr, decode_datagram, encode_datagram, validate_datagram_budget,
 };
 use quinn::{ClientConfig, Endpoint, crypto::rustls::QuicClientConfig};
 use rustls::pki_types::CertificateDer;
@@ -56,6 +55,13 @@ const CAM_REFRESH_ETHERTYPE: u16 = 0x88b5;
 const CAM_REFRESH_PAYLOAD: &[u8] = b"lanparty-cam-refresh";
 #[cfg(target_os = "linux")]
 const MIN_ETHERNET_FRAME_WITHOUT_FCS: usize = 60;
+#[cfg(target_os = "linux")]
+const LAN_CAPTURE_BUFFER_LEN: usize = u16::MAX as usize;
+#[cfg(target_os = "linux")]
+const _: () = assert!(
+    LAN_CAPTURE_BUFFER_LEN
+        > lanparty_proto::MAX_STANDARD_ETHERNET_FRAME_LEN + lanparty_proto::ETHERNET_HEADER_LEN
+);
 
 #[derive(Debug, Parser)]
 #[command(
@@ -658,7 +664,7 @@ fn gateway_frame_log_line(
 #[cfg(target_os = "linux")]
 async fn read_lan_ethernet(packet_socket: &AsyncFd<PacketSocket>) -> Result<Bytes> {
     loop {
-        let mut buffer = vec![0; MAX_STANDARD_ETHERNET_FRAME_LEN];
+        let mut buffer = vec![0; LAN_CAPTURE_BUFFER_LEN];
         let mut guard = packet_socket
             .readable()
             .await