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
+}

assets/shaders/xp_orb.gdshader.uid

@@ -0,0 +1 @@
+uid://ckjyt0mt0ngk

scenes/xp_orb.tscn

@@ -1,17 +1,31 @@
-[gd_scene load_steps=4 format=3 uid="uid://du4dqne3o0d5b"]
+[gd_scene load_steps=7 format=3 uid="uid://du4dqne3o0d5b"]
 
 [ext_resource type="Script" uid="uid://bx43us4qd2vpf" path="res://scenes/xp_orb.gd" id="1_me6n8"]
-[ext_resource type="Texture2D" uid="uid://c0g255217x10l" path="res://assets/sprites/xp_orb_small.png" id="2_me6n8"]
+[ext_resource type="Shader" uid="uid://ckjyt0mt0ngk" path="res://assets/shaders/xp_orb.gdshader" id="2_me6n8"]
+
+[sub_resource type="ShaderMaterial" id="ShaderMaterial_pkphv"]
+shader = ExtResource("2_me6n8")
+shader_parameter/orb_color = Color(0.2, 1, 0.5, 1)
+shader_parameter/glow_strength = 1.0
+shader_parameter/pulse_speed = 2.0
+
+[sub_resource type="Gradient" id="Gradient_me6n8"]
+colors = PackedColorArray(1, 1, 1, 1, 1, 1, 1, 1)
+
+[sub_resource type="GradientTexture2D" id="GradientTexture2D_hluwh"]
+gradient = SubResource("Gradient_me6n8")
+width = 12
+height = 12
 
 [sub_resource type="CircleShape2D" id="CircleShape2D_me6n8"]
-radius = 5.0
+radius = 4.0
 
 [node name="XPOrb" type="Node2D" groups=["pickup", "xp_orb"]]
 script = ExtResource("1_me6n8")
 
 [node name="Sprite2D" type="Sprite2D" parent="."]
-position = Vector2(-1, 1)
-texture = ExtResource("2_me6n8")
+material = SubResource("ShaderMaterial_pkphv")
+texture = SubResource("GradientTexture2D_hluwh")
 
 [node name="Area2D" type="Area2D" parent="."]
 collision_layer = 9