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.toml

@@ -31,9 +31,7 @@ hex = "0.4"
 clap = { version = "4.5", features = ["derive"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
-tun = { version = "0.8.5", features = ["async"] }
-libc = "0.2"
-tokio = { version = "1", features = ["rt", "macros", "signal", "sync", "net", "time", "process"] }
+tokio = { version = "1", features = ["rt", "macros", "signal", "sync", "net", "time", "process", "io-util"] }
 futures = "0.3"
 simple-dns = "0.11.2"
 socket2 = { version = "0.6.2", features = ["all"] }
@@ -41,10 +39,18 @@ tokio-socks = "0.5"
 
 rustables = { version = "0.8.7", optional = true }
 
+[target.'cfg(unix)'.dependencies]
+tun = { version = "0.8.5", features = ["async"] }
+libc = "0.2"
+
 [target.'cfg(target_os = "linux")'.dependencies]
 rtnetlink = "0.20.0"
 bluer = { version = "0.17", features = ["bluetoothd", "l2cap"], optional = true }
 
+[target.'cfg(windows)'.dependencies]
+wintun = "0.5"
+windows-service = "0.7"
+
 [package.metadata.deb]
 maintainer = "Johnathan Corgan <jcorgan@corganlabs.com>"
 copyright = "2026 Johnathan Corgan"