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/serve-frank-medium.sh

@@ -87,6 +87,9 @@ fi
 : "${SPEC_DRAFT_TYPE_V:=f16}"
 : "${SPEC_DRAFT_POLL:=1}"
 : "${SPEC_DRAFT_POLL_BATCH:=1}"
+: "${SPEC_NGRAM_N_MATCH:=24}"
+: "${SPEC_NGRAM_N_MIN:=48}"
+: "${SPEC_NGRAM_N_MAX:=64}"
 
 NO_MMPROJ="${NO_MMPROJ:-1}"
 
@@ -129,6 +132,16 @@ if [[ -n "$SPEC_TYPE" && "$SPEC_TYPE" != "none" ]]; then
     if [[ "$SPEC_TYPE" == *dspark* || ( "$SPEC_DRAFT_P_MIN" != "0.0" && "$SPEC_DRAFT_P_MIN" != "0" ) ]]; then
         SPEC_ARGS+=(--spec-draft-n-min "$SPEC_DRAFT_N_MIN" --spec-draft-p-min "$SPEC_DRAFT_P_MIN")
     fi
+    # ngram-mod is draftless: it needs no draft context and its window is its
+    # own (--spec-ngram-mod-*), so it can ride in the chain beside a model
+    # drafter. It also costs no VRAM -- a CPU hash table.
+    if [[ "$SPEC_TYPE" == *ngram* ]]; then
+        SPEC_ARGS+=(
+            --spec-ngram-mod-n-match "$SPEC_NGRAM_N_MATCH"
+            --spec-ngram-mod-n-min "$SPEC_NGRAM_N_MIN"
+            --spec-ngram-mod-n-max "$SPEC_NGRAM_N_MAX"
+        )
+    fi
     if [[ -n "$SPEC_DRAFT_DEVICE" ]]; then
         SPEC_ARGS+=(--spec-draft-device "$SPEC_DRAFT_DEVICE")
     fi