Fix sun quad square artifact with hard radial discard cutoff

Add hard discard at radius 0.35 so no fragment beyond that point
reaches the additive blend stage. Tighten disc, glow, and edge
fade to fit within the cutoff boundary.

assets/shaders/celestial.frag.glsl

@@ -27,17 +27,20 @@ void main() {
     vec2 uv = TexCoord - 0.5;
     float dist = length(uv);
 
-    // Hard disc with smooth edge
+    // Hard circular cutoff — nothing beyond radius 0.35
-    float disc = smoothstep(0.42, 0.35, dist);
+    if (dist > 0.35) discard;
 
-    // Soft glow that fades to zero well within quad bounds
+    // Hard disc with smooth edge
-    float glow = exp(-dist * dist * 32.0) * 0.5;
+    float disc = smoothstep(0.35, 0.28, dist);
+
+    // Soft glow confined within cutoff radius
+    float glow = exp(-dist * dist * 40.0) * 0.5;
 
     // Combine disc and glow
     float alpha = max(disc, glow) * push.intensity;
 
-    // Fade to zero well before quad edges
+    // Smooth fade to zero at cutoff boundary
-    float edgeFade = 1.0 - smoothstep(0.30, 0.38, dist);
+    float edgeFade = 1.0 - smoothstep(0.25, 0.35, dist);
     alpha *= edgeFade;
 
     vec3 color = push.celestialColor.rgb;