FSR2: selective clamp, tonemapped accumulation, terrain load radius 3

- Selective neighborhood clamp: only modify history when there's actual
  motion or disocclusion — static pixels pass history through untouched,
  preventing jitter-chasing from the shifting variance box
- Tonemapped accumulation: Reinhard tonemap before blend compresses bright
  edges so they don't disproportionately cause jitter
- Jitter-aware sample weighting: blend 3-20% based on sample proximity
- Soft MV dead zone: smoothstep instead of step avoids spatial discontinuity
- Aggressive velocity response: 30%/px motion, 50% cap, 80% disocclusion
- Terrain loading: radius 3 (49 tiles) to prevent spawn hitches,
  processOneReadyTile for smooth progress bar updates

src/core/application.cpp

@@ -4042,7 +4042,7 @@ void Application::loadOnlineWorldTerrain(uint32_t mapId, float x, float y, float
             // then restore the full radius after entering the game.
             // This matches WoW's behavior: load quickly, stream the rest in-game.
             const int savedLoadRadius = 4;
-            terrainMgr->setLoadRadius(2);   // 5x5=25 tiles — balance between spawn hitches and load time
+            terrainMgr->setLoadRadius(3);   // 7x7=49 tiles — prevents hitches on spawn
             terrainMgr->setUnloadRadius(7);
 
             // Trigger tile streaming for surrounding area