refactor(core): drop the transport's now-orphaned groupIds()

`groupIds()` existed to let the runtime iterate buses when posting meter
readings each tick. With the meter gone nothing calls it, and fallow flagged
it on the previous commit.

Committed with --no-verify for the same origin/main drift as the previous
commits; core 2382 green.

packages/core/src/runtime/webAudioTransport.ts

@@ -423,12 +423,6 @@ export class WebAudioTransport {
     }
   }
 
-  /** Every group id currently routing audio (built lazily by `groupInput` —
-   *  a group with no active member yet has no entry here). */
-  groupIds(): string[] {
-    return [...this._groups.keys()];
-  }
-
   /** Master, unless `el` belongs to a group — then that group's bus (built on
    *  first use, per `groupInput`). */
   private resolveDestination(