Release v0.5.0: isolate screenshot editor and support manifest-based updates

src/screenshot.h

@@ -2,6 +2,14 @@
 #include "platform.h"
 namespace ks
 {
+// Only the on-demand screenshot process owns this state; no resident App dependency.
+struct ScreenshotHost {
+    HINSTANCE instance{};
+    HWND window{};
+    unsigned windows{};
+};
+inline constexpr UINT ScreenshotRequest=WM_APP+71, ScreenshotIdle=WM_APP+72;
+void showScreenshot(ScreenshotHost &);
 enum class LineStyle { Solid, Dotted, Dashed, DashDot };
 enum class LineEnds { End, Start, Both, None };
 enum class ScreenshotShape { None, Rectangle, Circle, Ellipse };