frank-medium: ngram-mod is a legal chain member -- wire it and measure it

Unlike MTP, ngram-mod needs no draft context: common/speculative.cpp adds the
ngram types unconditionally while the model-backed drafters are gated on
ctx_dft != nullptr, so 'draft-dflash,ngram-mod' is structurally allowed (and the
config vector's fixed order -- ngram first, then the drafters -- makes CLI order
irrelevant).

The harness grows the two things this question needs: --configs, and an ECHO
shape. ngram-mod can only ever match text that is present in the prompt, so
measuring it on prose calls it useless and measuring DFlash on echo calls it
wasteful; both shapes now run, and the greedy text of each is compared across
configurations because speculation may not change answers.

scripts/frank-medium.env

@@ -175,6 +175,16 @@ 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.
+SPEC_NGRAM_N_MATCH=24
+SPEC_NGRAM_N_MIN=48
+SPEC_NGRAM_N_MAX=64
+
 # Text only: the server must not autoload a vision tower.  Vision stays on
 # frank-small (Dankclaw, RTX 3090), which is the fleet's image endpoint.
 NO_MMPROJ=1