add Windows support (Phase 1)

Cross-platform paths: config_dir() uses %APPDATA%, data_dir() uses
%PROGRAMDATA% on Windows. TLS cert directory uses data_dir() instead
of hardcoded /usr/local/var/numa. Windows DNS discovery via ipconfig.
Fixed cfg gates from not(macos) to explicit linux to prevent Linux
code compiling on Windows. Added Windows target to CI and release
workflows with zip packaging.

System integration (numa install/service) not yet supported on Windows
— users run numa.exe manually.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -26,3 +26,14 @@ jobs:
         run: cargo test
       - name: audit
         run: cargo install cargo-audit && cargo audit
+
+  check-windows:
+    runs-on: windows-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: dtolnay/rust-toolchain@stable
+      - uses: Swatinem/rust-cache@v2
+      - name: build
+        run: cargo build
+      - name: clippy
+        run: cargo clippy -- -D warnings