replayer: wire --max-inflight-sessions cap into replay loop (B2)

Trace-driven dispatch is preserved by default (semaphore=None when the
flag is not set), but operators can now cap concurrent sessions to
reproduce session-admission scenarios from earlier sweeps without
artificial time compression.

replayer/__main__.py

@@ -19,6 +19,9 @@ def main() -> None:
     p.add_argument("--model", type=str, default="default", help="Model name for API")
     p.add_argument("--concurrency-limit", type=int, default=2000,
                    help="Max concurrent HTTP requests (safety limit)")
+    p.add_argument("--max-inflight-sessions", type=int, default=None,
+                   help="Cap on concurrent sessions (None = unlimited; "
+                        "trace-driven dispatch otherwise)")
     p.add_argument("--request-timeout", type=float, default=600.0)
     p.add_argument("--request-limit", type=int, default=None,
                    help="Limit number of requests to replay")
@@ -38,6 +41,7 @@ def main() -> None:
         concurrency_limit=args.concurrency_limit,
         request_timeout_s=args.request_timeout,
         request_limit=args.request_limit,
+        max_inflight_sessions=args.max_inflight_sessions,
     )
 
     results = asyncio.run(replay_trace(config))