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/spv/shaders.manifest.json

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