Slow pollution decay 4x — BASE_DECAY_RATE 0.008 → 0.002

Air pollution half-life goes from ~48 sim cycles (~2 min at 1x) to
~192 cycles (~8 min). Ground and water pollution persist proportionally
longer. Pollution from furnaces, campfires, and acid rain now lingers
long enough to meaningfully damage regions rather than clearing in seconds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main/java/com/livingworld/modules/pollution/PollutionModule.java

@@ -42,7 +42,7 @@ public final class PollutionModule implements SimulationModule {
 
     public static final String MODULE_ID = "pollution";
 
-    private static final double BASE_DECAY_RATE        = 0.008;
+    private static final double BASE_DECAY_RATE        = 0.002;
     private static final double GROUND_TO_WATER_LEACH  = 0.005;
     private static final double WATER_QUALITY_IMPACT   = 0.05;
     private static final double CHANGE_THRESHOLD       = 0.01;