perf: track component and relationship counts per entity

Introduce component and relationship counters to optimize entity
destruction and component checks. This allows for short-circuiting
the destruction process for bare entities and provides an O(1)
check for `hasAnyComponent`.

src/world.ts

@@ -21,6 +21,8 @@ export class World {
   // ── Entity pools ──────────────────────────────────
   private _generations: number[] = [];
   private _free: number[] = [];
+  private _componentCounts: number[] = [];
+  private _relCounts: number[] = [];
 
   // ── Component storage ─────────────────────────────
   private _components = new Map<symbol, SparseSet<any>>();
@@ -52,6 +54,8 @@ export class World {
     if (this._free.length > 0) {
       const idx = this._free.pop()!;
       const gen = this._generations[idx];
+      this._componentCounts[idx] = 0;
+      this._relCounts[idx] = 0;
       const e = makeEntity(idx, gen);
       this._emit({ type: "spawned", entity: e });
       return e;
@@ -59,6 +63,8 @@ export class World {
 
     const idx = this._generations.length;
     this._generations.push(1);
+    this._componentCounts.push(0);
+    this._relCounts.push(0);
     const e = makeEntity(idx, 1);
     this._emit({ type: "spawned", entity: e });
     return e;
@@ -69,6 +75,14 @@ export class World {
     const idx = entityIndex(entity);
     if (!this._isAlive(idx, entity)) return;
 
+    // Short-circuit: truly bare entities have nothing to clean up
+    if (this._componentCounts[idx] === 0 && this._relCounts[idx] === 0) {
+      this._generations[idx]++;
+      this._free.push(idx);
+      this._emit({ type: "destroyed", entity });
+      return;
+    }
+
     // Clean relationships before components
     for (const [key] of this._relForward) {
       // Entity as source
@@ -99,6 +113,8 @@ export class World {
       dirty.delete(idx);
     }
 
+    this._componentCounts[idx] = 0;
+    this._relCounts[idx] = 0;
     this._generations[idx]++;
     this._free.push(idx);
 
@@ -114,10 +130,7 @@ export class World {
   hasAnyComponent(entity: Entity): boolean {
     const idx = entityIndex(entity);
     if (!this._isAlive(idx, entity)) return false;
-    for (const store of this._components.values()) {
-      if (store.has(idx)) return true;
-    }
-    return false;
+    return this._componentCounts[idx] > 0;
   }
 
   // ── Component operations ──────────────────────────
@@ -131,9 +144,12 @@ export class World {
     this._assertAlive(idx, entity);
 
     const store = this._getOrCreateStore(def);
+    const existed = store.has(idx);
     const value = { ...def.defaults, ...init };
     store.set(idx, value);
 
+    if (!existed) this._componentCounts[idx]++;
+
     this._emit({ type: "componentAdded", entity, component: def });
     return value;
   }
@@ -149,6 +165,7 @@ export class World {
     this._dirty.get(def._key)?.delete(idx);
 
     if (removed) {
+      this._componentCounts[idx]--;
       this._emit({ type: "componentRemoved", entity, component: def });
     }
   }
@@ -265,6 +282,9 @@ export class World {
     }
     rev.add(si);
 
+    this._relCounts[si]++;
+    this._relCounts[ti]++;
+
     this._emit({
       type: "relationshipAdded",
       source,
@@ -570,6 +590,9 @@ export class World {
       }
     }
 
+    this._relCounts[si]--;
+    this._relCounts[ti]--;
+
     this._emit({
       type: "relationshipRemoved",
       source,