perf: reduce packet buffer slicing churn (#2381)

* bench: add packet bytes extraction Criterion benchmark

Adds a Criterion benchmark under easytier/benches/ covering
ZCPacket::payload_bytes and tunnel_payload_bytes at 1280/4096-byte payload
sizes, using iter_batched so ZCPacket construction stays in the setup phase
and is excluded from the timed region.

- Register the [[bench]] entry in easytier/Cargo.toml.
- Document the bench and PACKET_BYTES_* env vars in benches/README.md.

* perf: reduce packet buffer slicing churn

Replace BytesMut::split_off with Buf::advance in ZCPacket bytes
extraction paths (payload_bytes, tunnel_payload_bytes, convert_type,
drop_foreign_header) and in TunZCPacketToBytes, and simplify the
copy_from_slice in new_from_payload.

When the buffer is in its unique (VEC) representation, split_off promotes
it to the shared (ARC) representation, allocating a Shared control block
and bumping the refcount on every call, and pins the buffer in shared
mode. advance only mutates the in-place ptr/len/cap fields, avoiding that
allocation/refcount churn on the TX hot path. The byte data itself is not
copied by either path.

easytier/benches/README.md

@@ -1,4 +1,47 @@
-# TX Throughput Benchmark
+# Benchmarks
+
+Criterion benchmarks for EasyTier hot paths.
+
+| Bench                       | What it measures                                                                 |
+| --------------------------- | -------------------------------------------------------------------------------- |
+| `tx_throughput`             | End-to-end TX injection path through `peer_manager::send_msg_by_ip`              |
+| `packet_bytes_extraction`   | `ZCPacket::payload_bytes` / `tunnel_payload_bytes` extraction (advance hot path)  |
+
+## Packet Bytes Extraction
+
+Criterion benchmark for `ZCPacket` bytes extraction — the methods touched by the
+`advance`-based slicing refactor. Measures `payload_bytes` and
+`tunnel_payload_bytes` at two payload sizes (1280, 4096). Setup
+(`ZCPacket::new_with_payload`) runs in the benchmark harness's preparation
+phase and is excluded from the timed region, so the numbers reflect only the
+extraction call.
+
+### Quick start
+
+```bash
+cargo bench --bench packet_bytes_extraction
+```
+
+Smoke run:
+
+```bash
+PACKET_BYTES_MEASUREMENT_SECS=2 \
+PACKET_BYTES_WARMUP_SECS=1 \
+PACKET_BYTES_SAMPLE_SIZE=10 \
+cargo bench --bench packet_bytes_extraction -- --quiet
+```
+
+### Environment variables
+
+| Variable                        | Default | Notes                        |
+| ------------------------------- | ------- | ---------------------------- |
+| `PACKET_BYTES_MEASUREMENT_SECS` | `10`    | Criterion `measurement_time` |
+| `PACKET_BYTES_WARMUP_SECS`      | `3`     | Criterion `warm_up_time`     |
+| `PACKET_BYTES_SAMPLE_SIZE`      | `10`    | Criterion `sample_size` (min 10) |
+
+---
+
+## TX Throughput Benchmark
 
 Criterion benchmark for EasyTier's TX injection path (`peer_manager::send_msg_by_ip`).