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
-    float disc = smoothstep(0.42, 0.35, dist);
+    // Hard circular cutoff — nothing beyond radius 0.35
+    if (dist > 0.35) discard;
 
-    // Soft glow that fades to zero well within quad bounds
-    float glow = exp(-dist * dist * 32.0) * 0.5;
+    // Hard disc with smooth edge
+    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
-    float edgeFade = 1.0 - smoothstep(0.30, 0.38, dist);
+    // Smooth fade to zero at cutoff boundary
+    float edgeFade = 1.0 - smoothstep(0.25, 0.35, dist);
     alpha *= edgeFade;
 
     vec3 color = push.celestialColor.rgb;