game: use shader for enemy on-hit visual

assets/shaders/hit_flash.gdshader

@@ -0,0 +1,8 @@
+shader_type canvas_item;
+
+uniform float flash_amount : hint_range(0.0, 1.0) = 0.0; // 0 = normal, 1 = full white
+
+void fragment() {
+    vec4 tex_color = texture(TEXTURE, UV);
+    COLOR = mix(tex_color, vec4(1.0, 1.0, 1.0, tex_color.a), flash_amount);
+}

assets/shaders/hit_flash.gdshader.uid

@@ -0,0 +1 @@
+uid://p7evv1wldgsa