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)

include/client.h

@@ -53,6 +53,7 @@ typedef struct Client {
 	struct wl_listener set_hints;
 #endif
 	unsigned int bw;
+	int corner_radius;
 	uint32_t tags;
 	int isfloating, isurgent, isfullscreen, isscratchpad;
 	uint32_t resize; /* configure serial of a pending resize */