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

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()
+}