feat(coach): ground replies in cited reference ranges

Surface the population reference bands behind the metrics a Coach turn
carries as a compact, citation-aware grounding block appended after the
SNAPSHOT. For each present metric the block states the headline band, its
source (blood pressure on the ESH 2023 line, US/ESC framings as context
only), and where the user's value sits via the shared four-state
placement contract.

Framed strictly as general guidance: the block opens and closes with a
not-a-diagnosis caveat, never overrides the existing refusal/dose/drug-
level safety discipline, and carries no commercial brand name. Glucose
respects the diabetes opt-in — the tighter ADA management goal only when
the user has declared it, otherwise the general non-diabetic band.

The block is deterministic and inspectable: built purely from the
reference-range backbone over the same scalars the snapshot already
surfaces, so it can never cite a value or band the snapshot does not
carry.

src/app/api/insights/chat/route.ts

@@ -393,9 +393,18 @@ The user's message answers this clarifying question from their self-context ques
 React briefly and personally to the answer; do not repeat the question and do not ask it again.
 `
     : "";
+  // v1.18.6 (W7) — citation-aware reference-range grounding for the metrics
+  // present in this snapshot. Deterministic + brand-free; framed as general
+  // guidance, never a diagnosis. Appended after the SNAPSHOT so it is fully
+  // inspectable and the model reads the published population bands + the
+  // user's placement. Omitted entirely when no present metric is covered by
+  // the reference backbone (the builder returns null).
+  const groundingBlock = snapshot.referenceGrounding
+    ? `\n${snapshot.referenceGrounding}\n`
+    : "";
   const userPrompt = `SNAPSHOT
 ${snapshot.snapshotJson || "(no metric data in this user's log yet)"}
-${guidedBlock}
+${groundingBlock}${guidedBlock}
 CONVERSATION
 ${transcript}