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).

runtime/mtp_serve.sh

@@ -13,6 +13,8 @@ MAX_NUM_SEQS="${MAX_NUM_SEQS:-3}"
 MAX_BATCHED_TOKENS="${MAX_BATCHED_TOKENS:-8192}"
 LOAD_FORMAT="${LOAD_FORMAT:-fastsafetensors}"
 PORT="${PORT:-8000}"
+# Reclaim the CUDA-graph memory over-estimate to KV (see serve.sh). Set =1 to restore.
+export VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS="${VLLM_MEMORY_PROFILER_ESTIMATE_CUDAGRAPHS:-0}"
 echo "[mtp] qwen3_5_mtp — backend=$BACKEND, num_speculative_tokens=$NSPEC, model=$MODEL"
 exec vllm serve "$MODEL" \
   --served-model-name qwen \