Fix foot splash particles being cleared immediately when wading

Ripples were cleared every frame the player wasn't swimming, which
destroyed foot splash particles the same frame they were spawned.
Now particles expire naturally via their lifetime.

src/rendering/swim_effects.cpp

@@ -552,7 +552,8 @@ void SwimEffects::update(const Camera& camera, const CameraController& cc,
         }
     } else {
         rippleSpawnAccum = 0.0f;
-        ripples.clear();
+        // Don't clear ripples — foot splash particles are added while wading
+        // (not swimming) and need to live out their lifetime.
     }
 
     // --- Bubble spawning ---