feat(cli): per-role --mem-fraction-static + use in E4-pressured

E4-v1 / v2 / pressured-v1 all failed to fire admission rejections in
this workload because the default 0.6 mem-fraction-static gives
288K-token kv_pool per decoder, more than enough to absorb the
50-session trace even at concurrency=32.

This commit adds:
  --decode-mem-fraction-static  (overrides per-decode SGLang arg)
  --prefill-mem-fraction-static (symmetric for completeness)

Plumbed via topology.{decode,prefill}_extra_server_args. The
pressured sweep now uses --decode-mem-fraction-static 0.4 which
shrinks decoder kv_pool to ~192K tokens — should force enough
admission rejections to actually exercise the D→P snapshot path.

scripts/sweep_e4_pressured.sh

@@ -6,8 +6,10 @@
 #   --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)
+#   --decode-mem-fraction-static 0.4
+#       Plumbed through cli.py → topology.decode_extra_server_args →
+#       launcher. Shrinks per-decode KV pool, forcing admit_direct_append
+#       to reject more often.
 #
 # Hypotheses (same as docs/E4_PROTOCOL_ZH.md but in a stressed regime):
 #   H1'  E4-pressured TTFT p99 ≤ E1 TTFT p99
@@ -75,6 +77,7 @@ uv run --no-sync python -m agentic_pd_hybrid.cli benchmark-live \
   --kvcache-migration-reject-threshold "$REJECT_THRESHOLD" \
   --kvcache-direct-max-uncached-tokens 8192 \
   --kvcache-load-floor-bonus "$LOAD_FLOOR_BONUS" \
+  --decode-mem-fraction-static "${DECODE_MEM_FRAC:-0.4}" \
   --enable-d-to-p-sync 2>&1 | tee -a "$LOG"
 
 run_dir=$(ls -td "$OUTPUT"/kvcache-centric-*/ 2>/dev/null | head -1)