test: add cross-platform build verification test

Verifies the codebase compiles for all supported platforms
(Linux, macOS, Windows, FreeBSD on amd64/arm64). This catches
cases where platform-specific code is called without providing
stubs for all platforms.

From PR #781.

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

cmd/gt/build_test.go

@@ -0,0 +1,57 @@
+package main
+
+import (
+	"os"
+	"os/exec"
+	"runtime"
+	"testing"
+)
+
+// TestCrossPlatformBuild verifies that the codebase compiles for all supported
+// platforms. This catches cases where platform-specific code (using build tags
+// like //go:build !windows) is called from platform-agnostic code without
+// providing stubs for all platforms.
+func TestCrossPlatformBuild(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping cross-platform build test in short mode")
+	}
+
+	// Skip if not running on a platform that can cross-compile
+	// (need Go toolchain, not just running tests)
+	if os.Getenv("CI") == "" && runtime.GOOS != "darwin" && runtime.GOOS != "linux" {
+		t.Skip("skipping cross-platform build test on unsupported platform")
+	}
+
+	platforms := []struct {
+		goos   string
+		goarch string
+		cgo    string
+	}{
+		{"linux", "amd64", "0"},
+		{"linux", "arm64", "0"},
+		{"darwin", "amd64", "0"},
+		{"darwin", "arm64", "0"},
+		{"windows", "amd64", "0"},
+		{"freebsd", "amd64", "0"},
+	}
+
+	for _, p := range platforms {
+		p := p // capture range variable
+		t.Run(p.goos+"_"+p.goarch, func(t *testing.T) {
+			t.Parallel()
+
+			cmd := exec.Command("go", "build", "-o", os.DevNull, ".")
+			cmd.Dir = "."
+			cmd.Env = append(os.Environ(),
+				"GOOS="+p.goos,
+				"GOARCH="+p.goarch,
+				"CGO_ENABLED="+p.cgo,
+			)
+
+			output, err := cmd.CombinedOutput()
+			if err != nil {
+				t.Errorf("build failed for %s/%s:\n%s", p.goos, p.goarch, string(output))
+			}
+		})
+	}
+}