feat(kvc): honor admission backpressure hints + structural event logging

Replay-side changes paired with the SGLang admission hint:

- DecodeResidencyState gains pause_until_s; admission probe parses
  recommended_pause_ms and updates the per-D pause window.
- _wait_for_decode_pause is invoked at request entry points
  (_invoke_router, _invoke_session_direct) so requests stall before
  hitting a saturated D instead of timing out via mooncake.
- New CLI flags: --enable-backpressure (default off, baseline preserved),
  --backpressure-max-pause-s (cap on per-request sleep, default 2s).

Structural instrumentation written under <run_dir>/structural/:
- admission-events.jsonl: every admission probe (RTT, queue_depth,
  pause_ms, available_tokens, evicted_count)
- backpressure-events.jsonl: every actual pause sleep
- session-d-binding.jsonl: per-request policy decision

Used to validate the structural claims documented separately.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/agentic_pd_hybrid/cli.py

@@ -245,6 +245,21 @@ def main() -> None:
             "Disable per-session detail in the pool timeseries (smaller files)."
         ),
     )
+    replay.add_argument(
+        "--enable-backpressure",
+        action="store_true",
+        help=(
+            "Honor recommended_pause_ms hints from D's admission endpoint. "
+            "When set, replay sleeps before issuing requests to a saturated D. "
+            "Default off — preserves baseline behavior."
+        ),
+    )
+    replay.add_argument(
+        "--backpressure-max-pause-s",
+        type=float,
+        default=2.0,
+        help="Cap on per-request backpressure sleep, regardless of D hint.",
+    )
 
     sample = subparsers.add_parser(
         "sample-sessions",
@@ -473,6 +488,19 @@ def main() -> None:
             "Disable per-session detail in the pool timeseries (smaller files)."
         ),
     )
+    benchmark.add_argument(
+        "--enable-backpressure",
+        action="store_true",
+        help=(
+            "Honor recommended_pause_ms hints from D's admission endpoint."
+        ),
+    )
+    benchmark.add_argument(
+        "--backpressure-max-pause-s",
+        type=float,
+        default=2.0,
+        help="Cap on per-request backpressure sleep, regardless of D hint.",
+    )
     benchmark.add_argument(
         "--sample-profile",
         choices=["default", "small-append"],
@@ -556,6 +584,8 @@ def main() -> None:
             kvcache_prefill_normal_priority=args.kvcache_prefill_normal_priority,
             pool_poll_interval_s=args.pool_poll_interval_s,
             pool_poll_include_sessions=not args.pool_poll_no_sessions,
+            enable_backpressure=args.enable_backpressure,
+            backpressure_max_pause_s=args.backpressure_max_pause_s,
         )
         results = asyncio.run(replay_trace(config))
         print(
@@ -700,6 +730,8 @@ def main() -> None:
                 ),
                 pool_poll_interval_s=args.pool_poll_interval_s,
                 pool_poll_include_sessions=not args.pool_poll_no_sessions,
+                enable_backpressure=args.enable_backpressure,
+                backpressure_max_pause_s=args.backpressure_max_pause_s,
                 sample_profile=args.sample_profile,
                 min_initial_input_tokens=args.min_initial_input_tokens,
                 max_initial_input_tokens=args.max_initial_input_tokens,