Refactor Cross function for platform-specific builds

Updated Cross function to accept specific platforms and added error handling for invalid platforms.

magefile.go

@@ -39,11 +39,13 @@ func BuildCLI() error {
 	return buildBinary("olcrtc", "./cmd/olcrtc", goos, goarch)
 }
 
-// Cross builds olcrtc for all supported platforms.
-func Cross() error {
+// Cross builds olcrtc for specified platforms or all if none specified.
+// Usage: mage cross [platforms...]
+// Example: mage cross linux/amd64 windows/amd64
+func Cross(platforms ...string) error {
 	mg.Deps(Deps)
 
-	targets := []struct{ os, arch string }{
+	allTargets := []struct{ os, arch string }{
 		{"linux", "amd64"},
 		{"linux", "arm64"},
 		{"windows", "amd64"},
@@ -55,12 +57,35 @@ func Cross() error {
 		{"openbsd", "arm64"},
 	}
 
+	var targets []struct{ os, arch string }
+	
+	if len(platforms) == 0 {
+		targets = allTargets
+	} else {
+		wanted := make(map[string]bool)
+		for _, p := range platforms {
+			wanted[p] = true
+		}
+		
+		for _, t := range allTargets {
+			key := t.os + "/" + t.arch
+			if wanted[key] {
+				targets = append(targets, t)
+			}
+		}
+		
+		if len(targets) == 0 {
+			return fmt.Errorf("no valid platforms specified. Available: linux/amd64, linux/arm64, windows/amd64, darwin/amd64, darwin/arm64, freebsd/amd64, freebsd/arm64, openbsd/amd64, openbsd/arm64")
+		}
+	}
+
 	for _, t := range targets {
 		if err := buildBinary("olcrtc", "./cmd/olcrtc", t.os, t.arch); err != nil {
 			return err
 		}
 	}
 
+	fmt.Printf("✅ Built %d platform(s)\n", len(targets))
 	return nil
 }