fix(gateway): drop over-budget LAN frames

The gateway may see full-sized frames from the physical LAN even when the room
uses a smaller tunnel MTU. With no overlay fragmentation, those frames cannot be
encoded into the negotiated QUIC datagram budget. The live bridge previously
propagated that budget error and stopped, which made one oversized LAN frame a
fatal condition.

Teach the gateway send path to return a send outcome. Normal direct callers
still get an error for over-budget sends, but the Linux bridge loop now records,
logs, and drops those frames with a DatagramBudget drop reason before continuing
with later traffic. Malformed local Ethernet still remains an error because that
indicates a broken local boundary rather than ordinary LAN traffic.

The gateway stats test now covers the extra drop, and the frame-log test covers
the new drop reason. README now documents that over-budget LAN frames are
counted, dropped, and logged instead of fragmented or killing the bridge.

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

Refs: PLAN.md No fragmentation for MVP

crates/lanparty-obs/src/lib.rs

@@ -36,6 +36,7 @@ pub enum DropReason {
     ControlPlaneEtherType,
     DhcpServerReply,
     Ipv6RouterAdvertisement,
+    DatagramBudget,
     UnknownDestination,
     RateLimit,
 }