Add Gio app shells and platform stubs (T-003)

app-modern/platform/windows/doc.go

@@ -1,2 +1,3 @@
-// Package windows contains modern Windows platform adapters.
+// Package windows provides modern Windows platform adapters and non-Windows
+// stubs for package-level tests.
 package windows

app-modern/platform/windows/platform.go

@@ -0,0 +1,17 @@
+package windows
+
+// Edition identifies the application build channel shown by the UI.
+type Edition string
+
+const EditionModern Edition = "Modern"
+
+// Platform is the minimal boundary for target-specific capabilities.
+type Platform interface {
+	OS() string
+	Edition() Edition
+}
+
+// New returns the platform implementation selected by build tags.
+func New() Platform {
+	return newPlatform()
+}

app-modern/platform/windows/platform_stub.go

@@ -0,0 +1,19 @@
+//go:build !windows
+
+package windows
+
+import "runtime"
+
+type platformStub struct{}
+
+func newPlatform() Platform {
+	return platformStub{}
+}
+
+func (platformStub) OS() string {
+	return runtime.GOOS
+}
+
+func (platformStub) Edition() Edition {
+	return EditionModern
+}

app-modern/platform/windows/platform_test.go

@@ -0,0 +1,13 @@
+package windows
+
+import "testing"
+
+func TestPlatformStubContract(t *testing.T) {
+	platform := New()
+	if platform.OS() == "" {
+		t.Fatal("OS() should not be empty")
+	}
+	if platform.Edition() != EditionModern {
+		t.Fatalf("Edition() = %q, want %q", platform.Edition(), EditionModern)
+	}
+}

app-modern/platform/windows/platform_windows.go

@@ -0,0 +1,17 @@
+//go:build windows
+
+package windows
+
+type platform struct{}
+
+func newPlatform() Platform {
+	return platform{}
+}
+
+func (platform) OS() string {
+	return "windows"
+}
+
+func (platform) Edition() Edition {
+	return EditionModern
+}