perf: yield between replayed history messages

A session/load of a long history sent every update from one macrotask when
the client read fast enough, starving other sessions and incoming requests.
The replay now shares the consumer's 8 ms budget and yields between
messages. The updates and their order are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/acp-agent.ts

@@ -7992,6 +7992,10 @@ export class ClaudeAcpAgent {
     };
 
     const replayMessage = async (message: SessionMessage): Promise<void> => {
+      // A long history is sent in order, but with event-loop turns in between,
+      // so other sessions and incoming requests are not starved meanwhile.
+      const pause = messageLoopYielder.maybeYield();
+      if (pause) await pause;
       if (pending?.stopped || isReplayHiddenMetaMessage(message)) {
         return;
       }