Add isEmpty() helpers across all modules with clearable state.

Seventh expansion pass: domain-specific isEmpty() after moduleSnapshot on every module that has clearAll(), using existing count/snapshot helpers (async variants for temporal, spatial, semantic-vector, and reactive-state indexes).

Co-authored-by: Cursor <[email protected]>

network-stack/hyper-p2p-overlay-topology/index.js

@@ -100,6 +100,10 @@ class HyperP2POverlayTopology extends EventEmitter {
     return this.snapshot()
   }
 
+  isEmpty () {
+    return this.snapshot().degree === 0
+  }
+
   getStats () {
     return { ...this._stats, degree: this._neighbors.size, protocol: PROTOCOL }
   }