feat(client): add TAP-Windows adapter crate

The Windows client still needs a real Ethernet TAP boundary before it can pump
frames between the adapter and the relay session. Keep that OS-specific surface
separate from the QUIC client state by adding a `lanparty-client-tap` crate.

The new crate discovers TAP-Windows6 adapters through the Windows network
adapter registry class, validates `tap0901` component ids, constructs the
`\\.\Global\{NetCfgInstanceId}.tap` device path, opens the TAP device handle,
and exposes blocking Ethernet frame read/write helpers. It also wraps the
TAP-Windows IOCTLs for media status, driver MAC, and driver MTU.

This does not wire the TAP crate into `lanparty-client-win` yet and does not
attempt route protection. The value of this slice is the target-checkable OS
boundary that the next client pump can depend on.

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

Refs: PLAN.md Windows TAP client

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

@@ -0,0 +1,145 @@
+//! TAP-Windows6 adapter discovery and raw Ethernet frame I/O.
+//!
+//! This crate deliberately stays below the relay session layer. It only knows
+//! how to find and open an installed TAP-Windows6 Ethernet adapter; the Windows
+//! client binary owns when to connect it to QUIC and how to protect routes.
+
+use anyhow::{Result, bail};
+
+pub const TAP_COMPONENT_ID: &str = "tap0901";
+pub const TAP_ADAPTER_KEY: &str =
+    r"SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002BE10318}";
+pub const TAP_DEVICE_PREFIX: &str = r"\\.\Global\";
+pub const TAP_DEVICE_SUFFIX: &str = ".tap";
+const FILE_DEVICE_UNKNOWN: u32 = 0x0000_0022;
+const METHOD_BUFFERED: u32 = 0;
+const FILE_ANY_ACCESS: u32 = 0;
+const TAP_IOCTL_GET_MAC_REQUEST: u32 = 1;
+const TAP_IOCTL_GET_MTU_REQUEST: u32 = 3;
+const TAP_IOCTL_SET_MEDIA_STATUS_REQUEST: u32 = 6;
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub struct TapAdapterInfo {
+    instance_id: String,
+    component_id: String,
+}
+
+impl TapAdapterInfo {
+    pub fn new(instance_id: impl Into<String>, component_id: impl Into<String>) -> Result<Self> {
+        let instance_id = instance_id.into();
+        if instance_id.trim().is_empty() {
+            bail!("TAP adapter instance id cannot be empty");
+        }
+
+        let component_id = component_id.into();
+        if !is_tap_component_id(&component_id) {
+            bail!("unsupported TAP adapter component id {component_id:?}");
+        }
+
+        Ok(Self {
+            instance_id,
+            component_id,
+        })
+    }
+
+    #[must_use]
+    pub fn instance_id(&self) -> &str {
+        &self.instance_id
+    }
+
+    #[must_use]
+    pub fn component_id(&self) -> &str {
+        &self.component_id
+    }
+
+    #[must_use]
+    pub fn device_path(&self) -> String {
+        tap_device_path(&self.instance_id)
+    }
+}
+
+#[must_use]
+pub fn is_tap_component_id(component_id: &str) -> bool {
+    let id = component_id.trim();
+    id.eq_ignore_ascii_case(TAP_COMPONENT_ID)
+        || id.eq_ignore_ascii_case(concat!("root\\", "tap0901"))
+}
+
+#[must_use]
+pub fn tap_device_path(instance_id: &str) -> String {
+    format!("{TAP_DEVICE_PREFIX}{instance_id}{TAP_DEVICE_SUFFIX}")
+}
+
+#[must_use]
+pub const fn tap_control_code(request: u32) -> u32 {
+    (FILE_DEVICE_UNKNOWN << 16) | (FILE_ANY_ACCESS << 14) | (request << 2) | METHOD_BUFFERED
+}
+
+#[must_use]
+pub const fn tap_ioctl_get_mac() -> u32 {
+    tap_control_code(TAP_IOCTL_GET_MAC_REQUEST)
+}
+
+#[must_use]
+pub const fn tap_ioctl_get_mtu() -> u32 {
+    tap_control_code(TAP_IOCTL_GET_MTU_REQUEST)
+}
+
+#[must_use]
+pub const fn tap_ioctl_set_media_status() -> u32 {
+    tap_control_code(TAP_IOCTL_SET_MEDIA_STATUS_REQUEST)
+}
+
+#[cfg(windows)]
+mod windows;
+
+#[cfg(windows)]
+pub use windows::{TapAdapter, available_adapters, open_first_adapter};
+
+#[cfg(not(windows))]
+pub fn available_adapters() -> Result<Vec<TapAdapterInfo>> {
+    bail!("TAP-Windows6 adapter discovery is only available on Windows");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn identifies_tap_windows_component_ids() {
+        assert!(is_tap_component_id("tap0901"));
+        assert!(is_tap_component_id("TAP0901"));
+        assert!(is_tap_component_id("root\\tap0901"));
+        assert!(!is_tap_component_id("wintun"));
+        assert!(!is_tap_component_id("tap0801"));
+    }
+
+    #[test]
+    fn builds_tap_device_path() {
+        assert_eq!(
+            tap_device_path("{01234567-89AB-CDEF-0123-456789ABCDEF}"),
+            r"\\.\Global\{01234567-89AB-CDEF-0123-456789ABCDEF}.tap"
+        );
+    }
+
+    #[test]
+    fn computes_tap_ioctl_codes() {
+        assert_eq!(tap_ioctl_get_mac(), 0x0022_0004);
+        assert_eq!(tap_ioctl_get_mtu(), 0x0022_000c);
+        assert_eq!(tap_ioctl_set_media_status(), 0x0022_0018);
+    }
+
+    #[test]
+    fn validates_adapter_info() {
+        let info =
+            TapAdapterInfo::new("{01234567-89AB-CDEF-0123-456789ABCDEF}", "tap0901").unwrap();
+
+        assert_eq!(info.component_id(), "tap0901");
+        assert_eq!(
+            info.device_path(),
+            r"\\.\Global\{01234567-89AB-CDEF-0123-456789ABCDEF}.tap"
+        );
+        assert!(TapAdapterInfo::new("", "tap0901").is_err());
+        assert!(TapAdapterInfo::new("{01234567-89AB-CDEF-0123-456789ABCDEF}", "wintun").is_err());
+    }
+}