frank-medium: chain DFlash2 with ngram-mod -- measured free where it cannot match

ngram-mod is draftless (added to the spec config vector unconditionally, while
every model-backed drafter needs ctx_dft), so unlike MTP it can share the
process with DFlash2. Chained at the fleet's 48/64 window, over three shapes:

  prose 1k/512      dflash 43.77 | ngram 17.72 (0 drafts) | chain 43.65
  echo 1.5k/400     dflash 76.82 | ngram 102.50          | chain 200.58
  structured/400    dflash 91.08 | ngram 17.80 (0 drafts) | chain 91.22
  identity          dflash 70.37 | ngram 17.77           | chain 70.40

On prose the chain is byte-identical to the drafter alone (same 119 drafts/49
accepts) -- that is what makes it a safe default -- and on prompt-echo traffic
it is 2.6x. VRAM is unchanged: the matcher is a 64 MiB host-RAM hash table.

The window sweep says keep the long one: 48/64 beats 16/32 (161.0) and 8/16
(113.8) on echo, and shortening it costs prose (43.9 -> 36.0) because the
matcher then proposes on ordinary fragments and the rejections still have to be
verified. That is the 'ngram hurts' result this fleet hit twice before.

Two documentation traps recorded with it: ngram-mod prints NO startup banner in
this build (its drafts are only visible behaviourally), and the harness's
'structured' shape is a negative control because its lines differ.

scripts/frank-medium.env

@@ -137,21 +137,24 @@ REASONING_BUDGET=-1
 CHAT_TEMPLATE_FILE=
 CHAT_TEMPLATE_KWARGS='{"enable_thinking":true,"preserve_thinking":true}'
 
-# Speculative decoding: the DFlash2 block-diffusion drafter.  Its GGUF carries
-# block_size 8, so at most 7 draft tokens per verification step (llama.cpp
-# asserts n_max <= block_size - 1); 7 is also what the drafter was published
-# with.  The drafter rides ROCm1, the card that ends up holding the target's
-# last layers -- the hidden states the proposal consumes.
-SPEC_TYPE=draft-dflash
+# Speculative decoding: the DFlash2 block-diffusion drafter PLUS the draftless
+# n-gram matcher. Measured against the drafter alone on 2026-09-16 (tables
+# below): the chain is byte-identical to DFlash2 by itself where n-grams cannot
+# match, and ~2.6x it where they can. DFlash2's GGUF carries block_size 8, so at
+# most 7 draft tokens per verification step (llama.cpp asserts
+# n_max <= block_size - 1); 7 is also what the drafter was published with. The
+# drafter rides ROCm1, the card that ends up holding the target's last layers.
+SPEC_TYPE=draft-dflash,ngram-mod
 SPEC_DRAFT_N_MAX=7
 SPEC_DRAFT_NGL=999
 SPEC_DRAFT_DEVICE=ROCm1
 SPEC_DRAFT_TYPE_K=f16
 SPEC_DRAFT_TYPE_V=f16
 
-# MTP vs the drafter, MEASURED 2026-09-16 (temperature 0, 1k prompt / 512 out;
-# the autoregressive baseline and both MTP depths were run against the deployed
-# configuration in one session, same binary, same prompts):
+# MTP (the model's own head) vs the drafter -- MEASURED 2026-09-16 (temperature
+# 0, 1k prompt / 512 out; the autoregressive baseline and both MTP depths were
+# run against the deployed configuration in one session, same binary, same
+# prompts):
 #
 #   none            17.7 tok/s                     card1 26.07 GB
 #   draft-mtp  n=3  32.4 tok/s  58 % accepted       card1 29.35 GB
@@ -175,12 +178,29 @@ SPEC_DRAFT_TYPE_V=f16
 # the target model ..."), so a chain of the two is a structural dead end, not a
 # tuning question. See serverAppConfigs-docs/LLM/frank-medium-llamacpp-dflash2.md.
 
-# ngram-mod: the draftless n-gram matcher (CPU hash table, no VRAM, no model
-# file). It is a legitimate second chain member -- unlike MTP it needs no draft
-# context, so `--spec-type draft-dflash,ngram-mod` is structurally allowed. The
-# window below is the fleet's tuned one (n_min/n_max 48/64 match long echoes,
-# which is the only traffic where n-grams pay). Measured against DFlash2 on
-# 2026-09-16 -- see the runbook for the verdict before enabling it.
+# ngram-mod: the draftless n-gram matcher (CPU hash table in host RAM, no VRAM,
+# no model file, no draft context). It joins the chain unconditionally --
+# unlike MTP, which needs a draft context and therefore cannot share one with
+# DFlash2. Verified on the loaded server: with ngram-mod as the ONLY spec type,
+# a prose prompt produces ZERO drafts (plain autoregressive) while a
+# prompt-echo prompt accepts 100 % of them, so its drafts are unmistakable.
+#
+# WINDOW MEASURED 2026-09-16 (chain at each window, decode tok/s):
+#
+#   n_min/n_max    prose 1k/512   prompt-echo 1.5k/400
+#   48/64          43.9           200.6     <- deployed
+#   16/32          36.0           161.0
+#    8/16          36.2           113.8
+#
+# The long window is the right one, and not only because it wins on echo: a
+# SHORT window makes the matcher propose on short fragments of ordinary prose,
+# where the proposals are rejected and the verification still costs -- which is
+# the "ngram hurts" result the fleet measured on other payloads twice before.
+# With 48/64 the chain is byte-identical to DFlash2 alone on prose (same 119
+# drafts / 49 accepted) and on the identity prompt (70/53).
+#
+# Do not shorten these without re-running scripts/bench-spec-ab.py: the
+# no-penalty property is what makes the chain safe as a default.
 SPEC_NGRAM_N_MATCH=24
 SPEC_NGRAM_N_MIN=48
 SPEC_NGRAM_N_MAX=64