Add size() cardinality helper across modules (eighth pass).

Standardize a size() method after isEmpty() on every module with clearAll(), delegating to domain counts or snapshot fields so callers can query occupancy without parsing snapshots.

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

messaging-streams/hyper-p2p-stream-multiplex/index.js

@@ -143,6 +143,10 @@ class HyperP2PStreamMultiplex extends EventEmitter {
     return this.snapshot().open === 0
   }
 
+  size () {
+    return this.snapshot().open
+  }
+
   openStreamBatch (ids) {
     if (!Array.isArray(ids)) throw new Error('ids array required')
     return ids.map((id) => this.openStream(id))