kv: recover dense pool 376k->427k (+13%) at the same 0.82 headroom

Reclaims over-reserved memory rather than spending headroom, so it honors
the conservative 0.82 / 14 GB-OS-reserve choice (still ~15 GiB free under
peak load) while closing most of the gap to the dflash pool (456k):

- int8 lm-head: build int8 on first warmup forward, then free the dead bf16
  copy (~1.4 GiB) + empty_cache() so the block lands in the KV pool. Safe:
  the DFlash drafter shares the one int8 lm_head module (verified one int8
  build; drafter ckpt has no lm_head/embed) and tie_word_embeddings=False,
  so bf16 has no reader (bias fallback = int8-GEMV + add-bias).
  SPARK_KEEP_BF16_LMHEAD=1 restores keep-bf16.
- VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS=0 in serve.sh/mtp_serve.sh:
  returns the CUDA-graph over-estimate (~0.6 GiB; actual capture ~0.14 GiB
  from the wide 0.82 headroom). Overridable with =1.
- scripts/stress_mem.sh: drive the concurrency bank while sampling host RAM
  to prove a gpu-mem setting survives peak load without swapping.

Validated: pool 426,610 tokens, coherent output, drafter acceptance 4-12,
no new swap, throughput unchanged. max-batched-tokens 8192->4096 tested as
a KV lever and rejected (only +2.5k tokens, costs prefill speed).

scripts/stress_mem.sh

@@ -0,0 +1,33 @@
+#!/bin/bash
+# stress_mem.sh — drive the server with the concurrency bank while sampling host
+# memory, to prove a gpu-memory-utilization setting survives peak load WITHOUT
+# swapping (the failure mode that hard-freezes the Spark). Reports min available
+# RAM and max swap observed across the run.
+#
+#   usage: stress_mem.sh [LEVELS]   (default "1,2,3")
+# Run on the box; the server (container qwen-spark) must be READY on :8000.
+set -uo pipefail
+LEVELS="${1:-1,2,3}"
+HERE="$(cd "$(dirname "$0")" && pwd)"
+LOG=/tmp/stress_memlog.$$
+
+# baseline
+echo "[stress] swap before:"; free -m | awk '/Swap:/{print "  swap_used_MiB",$3}'
+echo "[stress] avail before:"; free -m | awk '/Mem:/{print "  avail_MiB",$7}'
+
+# background sampler: timestamp, avail MiB, swap-used MiB, every 1s
+( for i in $(seq 1 900); do
+    free -m | awk -v t="$i" '/Mem:/{a=$7} /Swap:/{s=$3} END{print t, a, s}'
+    sleep 1
+  done ) > "$LOG" 2>/dev/null &
+SPID=$!
+
+echo "[stress] running conc_workloads --levels $LEVELS ..."
+python3 "$HERE/conc_workloads.py" --levels "$LEVELS"
+RC=$?
+
+kill "$SPID" 2>/dev/null
+echo "[stress] === memory envelope during load ==="
+awk 'NF>=3 {if(min==""||$2<min)min=$2; if($3>maxs)maxs=$3} END{print "  min_avail_MiB", min, " max_swap_MiB", maxs+0}' "$LOG"
+rm -f "$LOG"
+echo "[stress] conc_workloads exit=$RC"