Trash maze MVP (#79)

Co-authored-by: Michal Pikulski <michal@foolhardyhorizons.com>
Co-authored-by: Michal Pikulski <michal.a.pikulski@gmail.com>
Reviewed-on: #79

Assets/Shaders/TrashMaze/RevealStamp.shader

@@ -0,0 +1,64 @@
+Shader "Hidden/TrashMaze/RevealStamp"
+{
+    Properties
+    {
+        _StampPos ("Stamp Position", Vector) = (0.5, 0.5, 0, 0)
+        _StampRadius ("Stamp Radius", Float) = 0.2
+    }
+    
+    SubShader
+    {
+        Tags { "Queue"="Overlay" "RenderType"="Opaque" }
+        
+        // Additive blend to accumulate stamps
+        Blend One One
+        ZTest Always
+        ZWrite Off
+        Cull Off
+        
+        Pass
+        {
+            HLSLPROGRAM
+            #pragma vertex vert
+            #pragma fragment frag
+            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
+            
+            struct Attributes
+            {
+                float4 positionOS : POSITION;
+                float2 uv : TEXCOORD0;
+            };
+            
+            struct Varyings
+            {
+                float4 positionCS : SV_POSITION;
+                float2 uv : TEXCOORD0;
+            };
+            
+            float4 _StampPos;
+            float _StampRadius;
+            
+            Varyings vert(Attributes input)
+            {
+                Varyings output;
+                output.positionCS = TransformObjectToHClip(input.positionOS.xyz);
+                output.uv = input.uv;
+                return output;
+            }
+            
+            half4 frag(Varyings input) : SV_Target
+            {
+                // Calculate distance from stamp center
+                float dist = distance(input.uv, _StampPos.xy);
+                
+                // Binary circle: 1.0 inside radius, 0.0 outside
+                float alpha = dist < _StampRadius ? 1.0 : 0.0;
+                
+                // Return white with calculated alpha (additive blend accumulates)
+                return half4(alpha, alpha, alpha, alpha);
+            }
+            ENDHLSL
+        }
+    }
+}
+