feat(soak): core types + Deferred primitive

Establishes the Scenario/Session/Logger/DashboardReporter contracts
the rest of the harness builds on. Deferred is the building block
for RoomCoordinator's host→joiners handoff.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/soak/core/deferred.ts

@@ -0,0 +1,20 @@
+/**
+ * Promise deferred primitive — lets external code resolve or reject
+ * a promise. Used by RoomCoordinator for host→joiners handoff.
+ */
+
+export interface Deferred<T> {
+  promise: Promise<T>;
+  resolve(value: T): void;
+  reject(error: unknown): void;
+}
+
+export function deferred<T>(): Deferred<T> {
+  let resolve!: (value: T) => void;
+  let reject!: (error: unknown) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}