feat(proto): validate negotiated datagram budgets

PLAN.md keeps MVP traffic to one Ethernet frame per QUIC datagram with no
fragmentation. The relay already negotiates a datagram budget, but the client
and gateway send paths still relied on Quinn to reject oversized encoded
Ethernet datagrams.

Add a shared protocol validation helper for encoded datagram length versus the
negotiated QUIC budget. Thread the negotiated budget into client and gateway
send boundaries, reject oversized datagrams before send, and count those valid
but unsent frames as local drops.

Document the budget check in the workspace decomposition and gateway behavior.

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

Refs: PLAN.md

crates/lanparty-proto/src/overlay.rs

@@ -170,6 +170,8 @@ pub enum ProtoError {
     UnknownFrameType(u8),
     #[error("payload length {len} exceeds wire maximum {max}")]
     PayloadTooLarge { len: usize, max: usize },
+    #[error("encoded datagram length {len} exceeds negotiated QUIC datagram budget {max}")]
+    DatagramExceedsBudget { len: usize, max: usize },
     #[error("declared payload length {declared} does not match actual length {actual}")]
     PayloadLengthMismatch { declared: usize, actual: usize },
     #[error("Ethernet frame is too short: got {actual} bytes, need at least {minimum}")]
@@ -190,6 +192,20 @@ pub fn encode_datagram(
     Ok(datagram)
 }
 
+pub fn validate_datagram_budget(
+    datagram_len: usize,
+    max_datagram_size: usize,
+) -> Result<(), ProtoError> {
+    if datagram_len > max_datagram_size {
+        return Err(ProtoError::DatagramExceedsBudget {
+            len: datagram_len,
+            max: max_datagram_size,
+        });
+    }
+
+    Ok(())
+}
+
 pub fn decode_datagram(bytes: &[u8]) -> Result<OverlayPacket<'_>, ProtoError> {
     let header = OverlayHeader::decode(bytes)?;
     let payload = &bytes[OVERLAY_HEADER_LEN..];
@@ -281,4 +297,18 @@ mod tests {
             ProtoError::PayloadTooLarge { .. }
         ));
     }
+
+    #[test]
+    fn rejects_datagrams_over_negotiated_budget() {
+        let datagram = encode_datagram(FrameType::Ethernet, 1, 2, 0, &[1, 2, 3]).unwrap();
+
+        assert!(validate_datagram_budget(datagram.len(), datagram.len()).is_ok());
+        assert_eq!(
+            validate_datagram_budget(datagram.len(), datagram.len() - 1).unwrap_err(),
+            ProtoError::DatagramExceedsBudget {
+                len: datagram.len(),
+                max: datagram.len() - 1
+            }
+        );
+    }
 }