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))

replayer/replay.py

@@ -283,6 +283,11 @@ async def replay_trace(config: ReplayConfig) -> list[RequestMetrics]:
     trace_span = latest_ts - earliest_ts
 
     request_sem = asyncio.Semaphore(config.concurrency_limit)
+    session_sem = (
+        asyncio.Semaphore(config.max_inflight_sessions)
+        if config.max_inflight_sessions and config.max_inflight_sessions > 0
+        else None
+    )
 
     sink = IncrementalMetricSink(config.output_path)
 
@@ -291,6 +296,9 @@ async def replay_trace(config: ReplayConfig) -> list[RequestMetrics]:
     qps = n_requests / trace_span if trace_span > 0 else 0
     logger.info("Replaying %d sessions (%d requests) over %.0fs (%.2f req/s)",
                 n_sessions, n_requests, trace_span, qps)
+    if session_sem is not None:
+        logger.info("Session admission cap: %d concurrent sessions",
+                    config.max_inflight_sessions)
 
     pre_metrics = await _snapshot_prefix_cache_metrics(config.endpoint_url)
     sweep_start = time.perf_counter()
@@ -313,6 +321,7 @@ async def replay_trace(config: ReplayConfig) -> list[RequestMetrics]:
                     request_sem=request_sem,
                     earliest_ts=earliest_ts, sweep_start=sweep_start,
                     sink=sink,
+                    session_sem=session_sem,
                 ))
                 for sid, turns in sessions
             ]