feat(experiments): E4-pressured sweep — force reseed via reject_threshold=1

E4-v1 produced 272 admission rejects (good) but zero /_snapshot HTTP
calls (bad, entrance gate bug fixed in e729d62). E4-v2 went the other
way: 0 rejects through 53% of trace, sync function never even called.

E4-pressured locks in the *fix-verified* code path by lowering
--kvcache-migration-reject-threshold from 3 to 1. After ONE
rejection the policy forces session migration, which lands in
_invoke_kvcache_seeded_router → _attempt_d_to_p_sync.

With the e729d62 fix in place, the d-to-p-sync.jsonl structural log
should now capture every prepare/dump/finalize decision so we can
forensic verify the D→P fast path is actually delivering KV bytes
to P's radix tree.

scripts/sweep_e4_pressured.sh

@@ -0,0 +1,87 @@
+#!/usr/bin/env bash
+# E4-pressured — same as E4 but tuned to force admission rejections so the
+# D→P snapshot fast-path actually fires.
+#
+# Key delta vs sweep_e4_kvc_v2_d_to_p_sync.sh:
+#   --kvcache-migration-reject-threshold 1   (was 3)
+#       After ONE rejection the policy migrates the session to a different
+#       D, which in turn triggers _invoke_kvcache_seeded_router → D→P sync.
+#   (rely solely on reject_threshold=1 for now; mem-fraction reduction
+#   would need extra_server_args plumbing which is out-of-scope here)
+#
+# Hypotheses (same as docs/E4_PROTOCOL_ZH.md but in a stressed regime):
+#   H1'  E4-pressured TTFT p99 ≤ E1 TTFT p99
+#   H2'  D→P snapshot succeeds for ≥ 20% of reseed-triggering requests
+#   H3'  D→P-pushed-then-cache-hit reduces re-prefill segment of reseed
+#        path TTFT measurably
+
+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=${OUTPUT:-outputs/e4p_kvc_v2_d_to_p_sync_pressured_50sess}
+IB_DEVICE=${IB_DEVICE:-mlx5_60}
+LOAD_FLOOR_BONUS=${LOAD_FLOOR_BONUS:-200}
+REJECT_THRESHOLD=${REJECT_THRESHOLD:-1}
+MEM_FRACTION=${MEM_FRACTION:-0.5}
+
+if [ ! -f "$TRACE" ]; then
+  echo "ERROR: trace not found at $TRACE" >&2
+  exit 1
+fi
+
+mkdir -p "$OUTPUT"
+LOG="$OUTPUT/sweep.log"
+
+log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG"; }
+
+log "=== E4-pressured: KVC v2 + RDMA + load-floor K=$LOAD_FLOOR_BONUS + D→P sync + reject_threshold=$REJECT_THRESHOLD + mem_fraction=$MEM_FRACTION ==="
+log "MODEL=$MODEL"
+log "TRACE=$TRACE ($(wc -l < $TRACE) requests)"
+log "OUTPUT=$OUTPUT"
+
+label=e4p_kvc_v2_d_to_p_sync_run1
+log "=== [E4p] $label starting ==="
+
+uv run --no-sync python -m agentic_pd_hybrid.cli benchmark-live \
+  --trace "$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 "$IB_DEVICE" \
+  --gpu-budget 4 \
+  --time-scale 1 \
+  --session-sample-rate 1.0 \
+  --target-duration-s 100000 \
+  --concurrency-limit 32 \
+  --timeout-s 1800 \
+  --request-timeout-s 300 \
+  --kvcache-admission-mode worker \
+  --kvcache-seed-min-turn-id 1 \
+  --kvcache-seed-max-inflight-decode -1 \
+  --kvcache-prefill-backup-policy release-after-transfer \
+  --kvcache-prefill-priority-eviction \
+  --kvcache-migration-reject-threshold "$REJECT_THRESHOLD" \
+  --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 "=== [E4p] $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