Keep the pillar reference formatter module-private

Only `pillarDetailLines` composes it, and the card reads the lines
rather than the individual band string.

src/components/insights/health-score-pillar-detail.ts

@@ -92,7 +92,7 @@ export function pillarObservedText(
 }
 
 /** The scored reference band, already converted into the account unit. */
-export function pillarReferenceText(
+function pillarReferenceText(
   pillar: ScorePillarResult,
   { t, glucoseUnit }: PillarDetailContext,
 ): string {