fix: remove duplicate zone weather, consolidate RNG, name star constants

- weather: remove duplicate setZoneWeather(15) for Dustwallow Marsh —
  second call silently overwrote the first with different parameters
- weather: replace duplicate static RNG in getRandomPosition() with
  shared weatherRng() to avoid redundant generator state
- starfield: extract day/night cycle thresholds into named constants
  (kDuskStart/kNightStart/kDawnStart/kDawnEnd/kFadeDuration)
- skybox: replace while-loop time wrapping with std::fmod — avoids
  O(n) iterations on large time jumps

src/rendering/skybox.cpp

@@ -202,10 +202,9 @@ void Skybox::update(float deltaTime) {
 }
 
 void Skybox::setTimeOfDay(float time) {
-    // Clamp to 0-24 range
-    while (time < 0.0f) time += 24.0f;
-    while (time >= 24.0f) time -= 24.0f;
-
+    // Wrap to [0, 24) range using fmod instead of iterative subtraction
+    time = std::fmod(time, 24.0f);
+    if (time < 0.0f) time += 24.0f;
     timeOfDay = time;
 }