feat(client): validate TAP Ethernet frame I/O

The future client pump should exchange Ethernet frames with TAP through a narrow
API, not raw byte reads and writes. Add TAP frame validation at the adapter
boundary so malformed or jumbo frames are rejected before they enter the relay
path.

Expose `TAP_FRAME_BUFFER_LEN` and `validate_tap_ethernet_frame`, then add
Windows helpers that read a TAP frame and validate it, or validate and fully
write an Ethernet frame. Raw read/write methods remain available for lower-level
adapter work.

Test Plan:
- cargo fmt --check
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- Windows-target cargo clippy for lanparty-client-tap with -D warnings
- git diff --check

Refs: PLAN.md one TAP Ethernet frame per datagram

crates/lanparty-client-tap/src/windows.rs

@@ -4,7 +4,7 @@ use std::{
     ptr::{null, null_mut},
 };
 
-use anyhow::{Context, Result};
+use anyhow::{Context, Result, bail};
 use lanparty_proto::MacAddr;
 use windows_sys::Win32::{
     Foundation::{
@@ -26,7 +26,7 @@ use windows_sys::Win32::{
 
 use crate::{
     TAP_ADAPTER_KEY, TapAdapterInfo, is_tap_component_id, tap_ioctl_get_mac, tap_ioctl_get_mtu,
-    tap_ioctl_set_media_status,
+    tap_ioctl_set_media_status, validate_tap_ethernet_frame,
 };
 
 #[derive(Debug)]
@@ -128,6 +128,13 @@ impl TapAdapter {
         Ok(bytes_read as usize)
     }
 
+    pub fn read_ethernet_frame(&self, buffer: &mut [u8]) -> Result<usize> {
+        let len = self.read_frame(buffer)?;
+        validate_tap_ethernet_frame(&buffer[..len])?;
+
+        Ok(len)
+    }
+
     pub fn write_frame(&self, frame: &[u8]) -> Result<usize> {
         let mut bytes_written = 0_u32;
         let ok = unsafe {
@@ -148,6 +155,16 @@ impl TapAdapter {
         Ok(bytes_written as usize)
     }
 
+    pub fn write_ethernet_frame(&self, frame: &[u8]) -> Result<()> {
+        validate_tap_ethernet_frame(frame)?;
+        let written = self.write_frame(frame)?;
+        if written != frame.len() {
+            bail!("partial TAP frame write: {written}/{}", frame.len());
+        }
+
+        Ok(())
+    }
+
     fn device_io_control(
         &self,
         code: u32,