perf(packet_def): replace split_off with advance (zero-copy) from #2381

Replace BytesMut::split_off with Buf::advance in packet extraction paths:
- convert_type: advance instead of split_off (TCP/UDP/WG hot path)
- payload_bytes, tunnel_payload_bytes: same
- convert_to_dummy_tunnel_packet: same
- virtual_nic TunZCPacketToBytes: same

split_off creates a second BytesMut sharing the same allocation (Arc
ref count churn). advance just moves the start pointer forward —
zero-copy, zero-alloc, no Arc operations.

Benchmark: pps neutral (glibc handles split_off pattern efficiently),
but eliminates Arc churn and is cleaner code.

Combined with #2385 safe initialization for best of both PRs.
All 53 packet/mpsc/forward_packet tests pass.

easytier/src/instance/virtual_nic.rs

@@ -24,7 +24,7 @@ use crate::{
 };
 
 use byteorder::WriteBytesExt as _;
-use bytes::{BufMut, BytesMut};
+use bytes::{Buf, BufMut, BytesMut};
 use cidr::{Ipv4Inet, Ipv6Inet};
 use futures::{SinkExt, Stream, StreamExt, lock::BiLock, ready};
 use pin_project_lite::pin_project;
@@ -180,12 +180,13 @@ impl ZCPacketToBytes for TunZCPacketToBytes {
         assert!(payload_offset >= 4);
 
         let ret = if self.has_packet_info {
-            let mut inner = inner.split_off(payload_offset - 4);
+            inner.advance(payload_offset - 4);
             let proto = infer_proto(&inner[4..]);
             self.fill_packet_info(&mut inner[0..4], proto)?;
             inner
         } else {
-            inner.split_off(payload_offset)
+            inner.advance(payload_offset);
+            inner
         };
 
         tracing::debug!(?ret, ?payload_offset, "convert zc packet to tun packet");

easytier/src/tunnel/packet_def.rs

@@ -1,9 +1,10 @@
+use bytes::Buf;
 use bytes::Bytes;
 use bytes::BytesMut;
+use zerocopy::byteorder::*;
 use zerocopy::AsBytes;
 use zerocopy::FromBytes;
 use zerocopy::FromZeroes;
-use zerocopy::byteorder::*;
 
 type DefaultEndian = LittleEndian;
 
@@ -638,7 +639,8 @@ impl ZCPacket {
     }
 
     pub fn payload_bytes(mut self) -> BytesMut {
-        self.inner.split_off(self.payload_offset())
+        self.inner.advance(self.payload_offset());
+        self.inner
     }
 
     pub fn peer_manager_header(&self) -> Option<&PeerManagerHeader> {
@@ -703,11 +705,12 @@ impl ZCPacket {
     }
 
     pub fn tunnel_payload_bytes(mut self) -> BytesMut {
-        self.inner.split_off(
+        self.inner.advance(
             self.packet_type
                 .get_packet_offsets()
                 .peer_manager_header_offset,
-        )
+        );
+        self.inner
     }
 
     pub fn convert_type(mut self, target_packet_type: ZCPacketType) -> Self {
@@ -753,7 +756,8 @@ impl ZCPacket {
             return Self::new_from_buf(buf, target_packet_type);
         }
 
-        Self::new_from_buf(self.inner.split_off(new_offset), target_packet_type)
+        self.inner.advance(new_offset);
+        Self::new_from_buf(self.inner, target_packet_type)
     }
 
     pub fn into_bytes(self) -> Bytes {