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/stack.py

@@ -66,6 +66,7 @@ def launch_pd_stack(
     timeout_s: float = 1200.0,
     router_request_timeout_s: float | None = None,
     include_router: bool = True,
+    naive_dp: bool = False,
 ) -> ManagedPdStack:
     run_dir.mkdir(parents=True, exist_ok=True)
     logs_dir = run_dir / "logs"
@@ -77,6 +78,7 @@ def launch_pd_stack(
         decode_policy=decode_policy,
         include_router=include_router,
         router_request_timeout_s=router_request_timeout_s,
+        naive_dp=naive_dp,
     )
 
     prefill_processes = [
@@ -195,6 +197,9 @@ def _build_process_env(topology: SingleNodeTopology) -> dict[str, str]:
         env["MC_MS_AUTO_DISC"] = "0"
         if topology.ib_device:
             env["MOONCAKE_DEVICE"] = topology.ib_device
+    elif topology.transfer_backend == "mooncake":
+        # Default to TCP when RDMA is not forced (e.g. loopback on same node)
+        env.setdefault("MOONCAKE_PROTOCOL", "tcp")
 
     repo_root = Path(__file__).resolve().parents[2]
     python_paths = [