game: use shader instead of sprite for xp orbs

assets/shaders/xp_orb.gdshader

@@ -0,0 +1,24 @@
+shader_type canvas_item;
+
+uniform vec4 orb_color : source_color = vec4(0.2, 1.0, 0.5, 1.0); // base orb color
+uniform float glow_strength : hint_range(0.0, 2.0) = 1.2;
+uniform float pulse_speed : hint_range(0.0, 10.0) = 2.0;
+
+void fragment() {
+    // Centered UV (0,0 in middle, -1..1 range)
+    vec2 uv = (UV - vec2(0.5)) * 2.0;
+    float dist = length(uv);
+
+    // Soft circular mask
+    float circle = smoothstep(0.8, 0.0, dist);
+
+    // Glow falloff
+    float glow = smoothstep(0.5, 0.0, dist) * glow_strength;
+
+    // Pulsing
+    float pulse = 0.5 + 0.5 * sin(TIME * pulse_speed);
+
+    vec3 color = orb_color.rgb * (circle + glow * pulse);
+
+    COLOR = vec4(color, circle); // alpha fades at edges
+}