feat(experiments): E4 protocol + sweep script — KVC + D→P vs naive PD

Pre-registers the E4 experiment that tests whether KVC + D→P RDMA
snapshot push beats the naive PD-disagg E1 baseline on the
inferact_50sess subset. Compared to E3 the only changed flag is
--enable-d-to-p-sync.

Three hypotheses (see docs/E4_PROTOCOL_ZH.md §2.3):
  H1 (main): E4 TTFT p99 ≤ E1 TTFT p99
  H2:       E4 reseed-mode TTFT < E3 reseed-mode TTFT
  H3:       E4 success count ≥ E3 success count

The full reseed → snapshot-push orchestration is wired in b9b0cf0
(_attempt_d_to_p_sync); the SGLang scheduler RPCs and the runtime
mem-leak fix are in 86412bb / a369722.

scripts/sweep_e4_kvc_v2_d_to_p_sync.sh

@@ -0,0 +1,82 @@
+#!/usr/bin/env bash
+# E4 — KVC v2 + RDMA + load-floor bonus + D→P snapshot push
+#
+# Identical to scripts/sweep_e3_kvc_v2_loadfloor_rdma.sh except for the
+# additional --enable-d-to-p-sync flag (which causes agentic to orchestrate
+# the snapshot RPCs on the reseed slow path, and stack.py to set
+# SGLANG_SNAPSHOT_LINK_ENABLE=1 per worker).
+#
+# See docs/E4_PROTOCOL_ZH.md for hypothesis matrix.
+
+set -euo pipefail
+cd "$(dirname "$0")/.."
+
+if [ -z "${CUDA_HOME:-}" ]; then
+  echo "ERROR: CUDA_HOME not set. Source scripts/setup_env.sh first." >&2
+  exit 1
+fi
+
+MODEL=${MODEL:-/mnt/models/Qwen/Qwen3-30B-A3B-Instruct-2507}
+TRACE=${TRACE:-outputs/inferact_50sess.jsonl}
+OUTPUT=${OUTPUT:-outputs/e4_kvc_v2_d_to_p_sync_50sess}
+IB_DEVICE=${IB_DEVICE:-mlx5_60}
+LOAD_FLOOR_BONUS=${LOAD_FLOOR_BONUS:-200}
+
+if [ ! -f "$TRACE" ]; then
+  echo "ERROR: trace not found at $TRACE" >&2
+  echo "Run: uv run --no-sync python scripts/sample_trace_subset.py --output $TRACE --sessions 50" >&2
+  exit 1
+fi
+
+mkdir -p "$OUTPUT"
+LOG="$OUTPUT/sweep.log"
+
+log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG"; }
+
+log "=== E4: KVC v2 + RDMA + load-floor K=$LOAD_FLOOR_BONUS + D→P sync ==="
+log "MODEL=$MODEL"
+log "TRACE=$TRACE ($(wc -l < $TRACE) requests)"
+log "OUTPUT=$OUTPUT"
+log "IB_DEVICE=$IB_DEVICE"
+log "MC_TRANSFER_TIMEOUT=${MC_TRANSFER_TIMEOUT:-default-30s}"
+
+label=e4_kvc_v2_d_to_p_sync_run1
+log ""
+log "=== [E4] $label starting ==="
+
+uv run --no-sync python -m agentic_pd_hybrid.cli benchmark-live \
+  --trace "$TRACE" \
+  --output-root "$OUTPUT" \
+  --mechanism kvcache-centric \
+  --policy kv-aware \
+  --model-path "$MODEL" \
+  --prefill-workers 1 --decode-workers 3 \
+  --prefill-tp-size 1 --decode-tp-size 1 \
+  --prefill-gpu-ids 0 --decode-gpu-ids 1,2,3 \
+  --transfer-backend mooncake \
+  --force-rdma --ib-device "$IB_DEVICE" \
+  --gpu-budget 4 \
+  --time-scale 1 \
+  --session-sample-rate 1.0 \
+  --target-duration-s 100000 \
+  --concurrency-limit 32 \
+  --timeout-s 1800 \
+  --request-timeout-s 300 \
+  --kvcache-admission-mode worker \
+  --kvcache-seed-min-turn-id 1 \
+  --kvcache-seed-max-inflight-decode -1 \
+  --kvcache-prefill-backup-policy release-after-transfer \
+  --kvcache-prefill-priority-eviction \
+  --kvcache-migration-reject-threshold 3 \
+  --kvcache-direct-max-uncached-tokens 8192 \
+  --kvcache-load-floor-bonus "$LOAD_FLOOR_BONUS" \
+  --enable-d-to-p-sync 2>&1 | tee -a "$LOG"
+
+run_dir=$(ls -td "$OUTPUT"/kvcache-centric-*/ 2>/dev/null | head -1)
+log "=== [E4] $label COMPLETED, artifacts at $run_dir ==="
+
+if [ -f "$run_dir/request-metrics.jsonl.summary.json" ]; then
+  cp "$run_dir/request-metrics.jsonl.summary.json" "$OUTPUT/${label}_summary.json"
+  cp "$run_dir/request-metrics.jsonl" "$OUTPUT/${label}_metrics.jsonl"
+  log "=== summary saved to $OUTPUT/${label}_summary.json ==="
+fi