feat: add Windows support to build system

- Add make build-windows for Windows AMD64
- Update make build-all to include Windows
- Update release script to package Windows binary as .zip
- Add Windows installation instructions to docs
- Windows binary: qfs-windows-amd64.exe (~17MB)

All platforms now supported:
- Linux AMD64 (.tar.gz)
- macOS Intel/ARM (.tar.gz)
- Windows AMD64 (.zip)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

scripts/release.sh

@@ -57,11 +57,19 @@ if [ -f "bin/qfs-darwin-arm64" ]; then
     echo -e "${GREEN}  ✓ qfs-${VERSION}-darwin-arm64.tar.gz${NC}"
 fi
 
+# Windows AMD64
+if [ -f "bin/qfs-windows-amd64.exe" ]; then
+    cd bin
+    zip -q "../${RELEASE_DIR}/qfs-${VERSION}-windows-amd64.zip" qfs-windows-amd64.exe
+    cd ..
+    echo -e "${GREEN}  ✓ qfs-${VERSION}-windows-amd64.zip${NC}"
+fi
+
 # Generate checksums
 echo ""
 echo -e "${YELLOW}→ Generating checksums...${NC}"
 cd "${RELEASE_DIR}"
-shasum -a 256 *.tar.gz > SHA256SUMS.txt
+shasum -a 256 *.tar.gz *.zip > SHA256SUMS.txt 2>/dev/null || shasum -a 256 * | grep -v SHA256SUMS > SHA256SUMS.txt
 cd ../..
 echo -e "${GREEN}  ✓ SHA256SUMS.txt${NC}"