Fix visible square behind sun by switching celestial to additive blending

Alpha blending caused faint quad edges to be visible against the sky
gradient. Additive blending correctly adds glow light without the
outline artifact. Edge fade starts earlier and discard threshold raised.

assets/shaders/celestial.frag.glsl

@@ -37,8 +37,8 @@ void main() {
     // Combine disc and glow
     float alpha = max(disc, glow) * push.intensity;
 
-    // Fade to zero at quad edges to prevent visible box
-    float edgeFade = 1.0 - smoothstep(0.4, 0.5, dist);
+    // Fade to zero well before quad edges
+    float edgeFade = 1.0 - smoothstep(0.35, 0.48, dist);
     alpha *= edgeFade;
 
     vec3 color = push.celestialColor.rgb;
@@ -56,6 +56,7 @@ void main() {
     float phaseShadow = smoothstep(-0.1, 0.1, phaseX);
     alpha *= mix(phaseShadow, 1.0, step(0.5, push.intensity));
 
-    if (alpha < 0.001) discard;
-    outColor = vec4(color, alpha);
+    if (alpha < 0.003) discard;
+    // Pre-multiply for additive blending: RGB is the light contribution
+    outColor = vec4(color * alpha, alpha);
 }