Hold first checkpoint in browser timing fixture

Why: The safe acknowledgement must be observed before fake findings arrive.

Evidence: Dev evidence receipt for ten browser scenarios and isolated PostgreSQL/Redis tests.

Impact: Changes only the task-owned zero-paid fixture delay.

Refs: U3 r0.5 implementation plan Task 4

internal/server/work_checkpoint_eval_test.go

@@ -139,7 +139,7 @@ func evalMechanismStream(ctx context.Context, model string, msgs []agentllm.Mess
 		} else {
 			content = "整理结束，日期与验收条件需要核对；完整范围以原件为准。"
 		}
-		if strings.Contains(sourceText, "[fixture:slow-stages]") {
+		if strings.Contains(sourceText, "[fixture:slow-stages]") || strings.Contains(sourceText, "[fixture:fast-ack]") {
 			time.Sleep(1500 * time.Millisecond)
 		} else {
 			time.Sleep(200 * time.Millisecond)