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/benchmark.py

@@ -45,6 +45,8 @@ class BenchmarkConfig:
     kvcache_prefill_normal_priority: int = 100
     pool_poll_interval_s: float = 0.0
     pool_poll_include_sessions: bool = True
+    enable_backpressure: bool = False
+    backpressure_max_pause_s: float = 2.0
     sample_profile: str = "default"
     min_initial_input_tokens: int | None = None
     max_initial_input_tokens: int | None = None
@@ -194,6 +196,8 @@ def run_live_benchmark(config: BenchmarkConfig) -> BenchmarkArtifacts:
             kvcache_prefill_normal_priority=config.kvcache_prefill_normal_priority,
             pool_poll_interval_s=config.pool_poll_interval_s,
             pool_poll_include_sessions=config.pool_poll_include_sessions,
+            enable_backpressure=config.enable_backpressure,
+            backpressure_max_pause_s=config.backpressure_max_pause_s,
         )
         if config.request_timeout_s is not None:
             replay_config = replace(
@@ -252,6 +256,8 @@ def run_live_benchmark(config: BenchmarkConfig) -> BenchmarkArtifacts:
                 ),
                 "pool_poll_interval_s": config.pool_poll_interval_s,
                 "pool_poll_include_sessions": config.pool_poll_include_sessions,
+                "enable_backpressure": config.enable_backpressure,
+                "backpressure_max_pause_s": config.backpressure_max_pause_s,
                 "sample_profile": config.sample_profile,
                 "min_initial_input_tokens": config.min_initial_input_tokens,
                 "max_initial_input_tokens": config.max_initial_input_tokens,