feat(effects): implement window rounded corners via scenefx

- Parse effects.windows.rounded/rounding from Lua config
- Add CfgEffectsWindows and CfgEffects structs to Config
- Switch to fx_renderer_create for scenefx support
- Replace 5 wlr/types/wlr_scene.h includes with scenefx variant
- Add config_get_corner_radius() helper (light=6px, heavy=14px)
- Apply corner_radii_top/bottom/left/right to 4 border rects
- Apply wlr_scene_buffer_set_corner_radius to surface buffers
- Disable on fullscreen, restore on exit
- Re-apply on resize and config hot-reload
- Add -Wno-extra-semi for scenefx header compatibility
- All targets compile cleanly (make test passes)

src/scratchpad.c

@@ -6,7 +6,7 @@
  * as a group, with monocle-style cycling within the scratchpad.
  */
 #include <wlr/types/wlr_output_layout.h>
-#include <wlr/types/wlr_scene.h>
+#include <scenefx/types/wlr_scene.h>
 #include <wlr/util/log.h>
 
 #include "client.h"