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.

packaging/Makefile

@@ -9,6 +9,7 @@
 #   make ipk        Build an OpenWrt .ipk package
 #   make aur        Build fips-git AUR package and validate with namcap
 #   make pkg        Build a macOS .pkg installer
+#   make zip        Build a Windows .zip package
 #   make all        Build deb and tarball (default)
 #   make clean      Remove deploy/ directory
 
@@ -16,7 +17,7 @@ SHELL := /bin/bash
 PACKAGING_DIR := $(dir $(abspath $(lastword $(MAKEFILE_LIST))))
 PROJECT_ROOT  := $(abspath $(PACKAGING_DIR)/..)
 
-.PHONY: all deb tarball ipk aur pkg clean
+.PHONY: all deb tarball ipk aur pkg zip clean
 
 all: deb tarball
 
@@ -35,5 +36,8 @@ aur:
 pkg:
 	@bash $(PACKAGING_DIR)/macos/build-pkg.sh
 
+zip:
+	@powershell -File $(PACKAGING_DIR)/windows/build-zip.ps1
+
 clean:
 	rm -rf $(PROJECT_ROOT)/deploy