Add Windows platform support (#45)

Gate platform-specific code behind cfg attributes and add full Windows
  support: TUN device via wintun, TCP control socket on localhost:21210,
  Windows Service lifecycle (--install-service/--uninstall-service/--service),
  CI build and test matrix, and packaging with ZIP builder and PowerShell
  service management scripts.

  Key changes:

  - Cargo.toml: move tun/libc/rtnetlink behind cfg(unix); add wintun and
    windows-service dependencies for Windows
  - upper/tun.rs: wintun-based TUN implementation with netsh configuration
    for IPv6 address, MTU, and fd00::/8 routing
  - control/mod.rs: split into unix_impl/windows_impl; Windows uses TCP on
    localhost:21210 with shared connection handler
  - bin/fips.rs: refactor main() into run_daemon() accepting a shutdown
    signal; add Windows Service support via windows-service crate
  - transport/udp/socket.rs: platform-gated modules; Windows uses
    tokio::net::UdpSocket (kernel drop count unavailable, returns 0)
  - transport/ethernet: gate to cfg(unix); add Windows stub types
  - config: platform-conditional default paths (socket, hosts) for Windows
  - CI: add windows-latest to build matrix and test-windows job with
    cargo-nextest
  - packaging/windows: build-zip.ps1, install-service.ps1,
    uninstall-service.ps1, and package-windows.yml workflow
  - README/docs: Windows build instructions, service management, and
    control socket platform differences

  Linux and macOS behavior is unchanged.

Cargo.lock

@@ -287,13 +287,33 @@ version = "1.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "b35204fbdc0b3f4446b89fc1ac2cf84a8a68971995d0bf2e925ec7cd960f9cb3"
 
+[[package]]
+name = "c2rust-bitfields"
+version = "0.18.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b43c3f07ab0ef604fa6f595aa46ec2f8a22172c975e186f6f5bf9829a3b72c41"
+dependencies = [
+ "c2rust-bitfields-derive 0.18.0",
+]
+
 [[package]]
 name = "c2rust-bitfields"
 version = "0.21.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dcee50917f9de1a018e3f4f9a8f2ff3d030a288cffa4b18d9b391e97c12e4cfb"
 dependencies = [
- "c2rust-bitfields-derive",
+ "c2rust-bitfields-derive 0.21.0",
+]
+
+[[package]]
+name = "c2rust-bitfields-derive"
+version = "0.18.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3cbc102e2597c9744c8bd8c15915d554300601c91a079430d309816b0912545"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 1.0.109",
 ]
 
 [[package]]
@@ -983,6 +1003,8 @@ dependencies = [
  "tracing",
  "tracing-subscriber",
  "tun",
+ "windows-service",
+ "wintun",
 ]
 
 [[package]]
@@ -3156,6 +3178,12 @@ dependencies = [
  "wezterm-dynamic",
 ]
 
+[[package]]
+name = "widestring"
+version = "1.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "72069c3113ab32ab29e5584db3c6ec55d416895e60715417b5b883a357c3e471"
+
 [[package]]
 name = "winapi"
 version = "0.3.9"
@@ -3193,6 +3221,26 @@ version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f0805222e57f7521d6a62e36fa9163bc891acd422f971defe97d64e70d0a4fe5"
 
+[[package]]
+name = "windows-service"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d24d6bcc7f734a4091ecf8d7a64c5f7d7066f45585c1861eba06449909609c8a"
+dependencies = [
+ "bitflags 2.10.0",
+ "widestring",
+ "windows-sys 0.52.0",
+]
+
+[[package]]
+name = "windows-sys"
+version = "0.52.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "282be5f36a8ce781fad8c8ae18fa3f9beff57ec1b52cb3de0789201425d9a33d"
+dependencies = [
+ "windows-targets 0.52.6",
+]
+
 [[package]]
 name = "windows-sys"
 version = "0.59.0"
@@ -3359,6 +3407,19 @@ dependencies = [
  "windows-sys 0.59.0",
 ]
 
+[[package]]
+name = "wintun"
+version = "0.5.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "da99be64b5aa3de869c16977994314d0759a698d9a73ab0a5b1d52e2282033ae"
+dependencies = [
+ "c2rust-bitfields 0.18.0",
+ "libloading 0.8.9",
+ "log",
+ "thiserror 1.0.69",
+ "windows-sys 0.52.0",
+]
+
 [[package]]
 name = "wintun-bindings"
 version = "0.7.34"
@@ -3366,7 +3427,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "27ae04d34b8569174e849128d2e36538329a27daa79c06ed0375f2c5d6704461"
 dependencies = [
  "blocking",
- "c2rust-bitfields",
+ "c2rust-bitfields 0.21.0",
  "futures",
  "libloading 0.9.0",
  "log",