feat: integrate MemoryDecay into app.js

- Import memory-decay component
- Initialize decay engine with SpatialMemory
- Update decay on each frame
- Recharge memories on access events
- Add decay stats display helper

app.js

@@ -4,6 +4,7 @@ import { RenderPass } from 'three/addons/postprocessing/RenderPass.js';
 import { UnrealBloomPass } from 'three/addons/postprocessing/UnrealBloomPass.js';
 import { SMAAPass } from 'three/addons/postprocessing/SMAAPass.js';
 import { SpatialMemory } from './nexus/components/spatial-memory.js';
+import { MemoryDecay } from './nexus/components/memory-decay.js';
 import { MemoryOptimizer } from './nexus/components/memory-optimizer.js';
 
 // ═══════════════════════════════════════════
@@ -709,6 +710,7 @@ async function init() {
   createAshStorm();
   SpatialMemory.init(scene);
   SpatialMemory.setCamera(camera);
+  MemoryDecay.init(SpatialMemory);
   updateLoad(90);
 
   loadSession();
@@ -2868,6 +2870,7 @@ function gameLoop() {
   // Project Mnemosyne - Memory Orb Animation
   if (typeof animateMemoryOrbs === 'function') {
     SpatialMemory.update(delta);
+    MemoryDecay.update(delta);
     animateMemoryOrbs(delta);
   }