decay: don't bump version for weight-only changes

Decay is metadata, not content. Bumping version caused unnecessary
log churn and premature cache invalidation.

Also disable auto-decay in scheduler — was causing version spam
and premature demotion of useful nodes.

src/store/ops.rs

@@ -197,8 +197,8 @@ impl Store {
             }
 
             // Only persist nodes whose weight actually changed
+            // Don't bump version — decay is metadata, not content change
             if (node.weight - old_weight).abs() > 1e-6 {
-                node.version += 1;
                 updated.push(node.clone());
                 decayed += 1;
             }