docs: KVC v1-v4 debug journey + raise session soft_cap to 16

Document the iterative debugging from v1 (broken KVC) through v4
(routing fixed + session cap raised), with code-level analysis of
the two main bugs encountered:

1. v2 root cause (mis-diagnosed previously as `allow_local_prefill`):
   `--policy default` for KVC mechanism caused replay's round-robin
   policy and the PD router's round-robin to diverge, sending requests
   with `session_params` to a D worker that did not have the session
   open. Resulted in 56-61% truncation with finish_reason
   "session id X does not exist".
   Fix: use `--policy kv-aware` (sweep_tp1_v3_kvaware.sh) so replay
   emits `x-smg-target-worker` and PD router uses consistent_hashing.

2. v3 new bottleneck: `pd-router-fallback-large-append-session-cap`
   dominated 52-65% of requests. Root cause was hardcoded
   `min(4, ...)` in `_decode_session_soft_cap`. With 7 D workers x 4
   sessions = 28 slots for 52 trace sessions, ~24 sessions starved
   permanently (bimodal direct-to-D rate of 0% or 99%).
   Fix: raise the cap to 16 (replay.py).

Also includes the v3 finding that direct-to-d-session path P50=0.495s
and TTFT P50=0.043s already beats the 8-way DP baseline (0.65s/0.093s)
- the KVC core mechanism works when fallback paths are avoided.

Files:
- docs/KVC_DEBUG_JOURNEY_V1_TO_V4.md: full journey + code location index
- docs/SWEBENCH_EXPERIMENT_{PROGRESS,RESULTS}.md: prior session notes
- scripts/sweep_tp1_v{2,3,4}*.sh: experiment driver scripts
- src/agentic_pd_hybrid/replay.py: cap 4 -> 16, audit fields
- src/agentic_pd_hybrid/pd_router.py: strip session_params from prefill
- src/agentic_pd_hybrid/metrics.py: truncated_request_count

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

src/agentic_pd_hybrid/benchmark.py

@@ -119,6 +119,8 @@ def run_live_benchmark(config: BenchmarkConfig) -> BenchmarkArtifacts:
     try:
         signal.signal(signal.SIGINT, _handle_termination)
         signal.signal(signal.SIGTERM, _handle_termination)
+        _mechanisms_with_router = {"pd-disaggregation", "kvcache-centric", "pd-colo"}
+        _naive_dp = config.mechanism_name == "pd-colo"
         if config.launch_stack:
             stack = launch_pd_stack(
                 topology=topology,
@@ -132,18 +134,19 @@ def run_live_benchmark(config: BenchmarkConfig) -> BenchmarkArtifacts:
                     else config.timeout_s
                 ),
                 include_router=(
-                    config.mechanism_name in {"pd-disaggregation", "kvcache-centric"}
+                    config.mechanism_name in _mechanisms_with_router
                 ),
+                naive_dp=_naive_dp,
             )
             router_url = (
                 stack.router_url
-                if config.mechanism_name in {"pd-disaggregation", "kvcache-centric"}
+                if config.mechanism_name in _mechanisms_with_router
                 else None
             )
         else:
             router_url = (
                 topology.router_url
-                if config.mechanism_name in {"pd-disaggregation", "kvcache-centric"}
+                if config.mechanism_name in _mechanisms_with_router
                 else None
             )