perf(sky): skip the night-sky lattice entirely while the day fade is zero

The enhanced sky multiplied the full 4-tier star lattice by a night
factor of 0.0 all day long. Early-out when the star layer's lighting
product says daytime, so the pack's night sky costs nothing outside
dusk-to-dawn. sky.frag.spv re-pinned.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/Shaders/sky.frag

@@ -191,6 +191,12 @@ void main() {
         // ~1.0 at noon. Stars at full strength only under a dark sky.
         float dayL = dot(vTint, vec3(0.299, 0.587, 0.114));
         float night = 1.0 - smoothstep(0.18, 0.45, dayL);
+        if (night < 0.004) {
+            // Daytime: the whole lattice would multiply to zero — skip it
+            // so the enhanced sky costs nothing outside dusk-to-dawn.
+            fragColor = vec4(0.0, 0.0, 0.0, 1.0);
+            return;
+        }
         vec3 sky = nightSky(normalize(vDir), uint(uParamB));
         // Additive pipeline (forced for this draw): rgb adds over the dome.
         fragColor = vec4(sky * night, 1.0);

src/AcDream.App/Rendering/Shaders/spv/shaders.manifest.json

@@ -316,7 +316,7 @@
         },
         {
           "stage": "frag",
-          "sourceSha256": "a6efd09f396b17ae6a19ffc4be400ac9b035056cbad26aee3447459cd98341f0",
+          "sourceSha256": "e7ccdb4d5fece2ceeb48eb43aaa827424df2b68614987eeb3dfb7880dff8ea83",
           "compiled": true
         }
       ]