Fold the side-effect tag through the shared helper

The three locale matchers were built in parallel and each grew its own
fold. Two of them are the same function: the side-effect tag fold and
foldForMatch agree character for character, so one now calls the other.

The third stays. foldLabel decomposes with NFKD, maps sharp s to ss and
joins with underscores; foldForMatch decomposes with NFD, leaves sharp s
alone and joins with spaces. That is a disagreement about who typed the
text, not an accident: one side of a lab comparison is a shipped label
retyped or prose a lab printed, where tolerating a different spelling
claims more than the fold can prove, while someone naming a metric to an
assistant is composing rather than reproducing. Both docblocks now say
so and name the other.

src/lib/i18n/fold-for-match.ts

@@ -18,6 +18,24 @@
  * Client-safe: no message bundles are imported here, so a client component may
  * use these. Building an index OVER the bundles is a server concern and lives
  * in `./shared-resolve`.
+ *
+ * ## Why there is a second fold, and when to reach for it
+ *
+ * `./localised-label-index` carries `foldLabel`, and the two disagree ON
+ * PURPOSE rather than by accident. That one decomposes with NFKD, maps "ß" to
+ * "ss" and "œ" to "oe", and joins with "_"; this one decomposes with NFD,
+ * leaves "ß" alone, and joins with spaces.
+ *
+ * The difference is who typed the text. Here the other side of the comparison
+ * is a shipped label a person retyped, or prose a lab printed, so tolerating a
+ * DIFFERENT SPELLING would be reaching beyond what the fold can honestly
+ * claim. There, someone is naming a metric to an assistant in their own words,
+ * and "Suessstoff" for "Süßstoff" is the same request; the wider fold earns
+ * its keep because the caller is composing, not reproducing.
+ *
+ * Keep them apart. Whichever you pick, apply it to BOTH sides of the
+ * comparison — a fold on one side only is an exact-string match in a
+ * disguise.
  */
 
 /** Curly, modifier and backtick apostrophes a keyboard or word processor emits. */

src/lib/medications/glp1-side-effect-tags.ts

@@ -21,6 +21,8 @@
  * therefore lives server-side in `./glp1-side-effect-tag-match`.
  */
 
+import { foldForMatch } from "@/lib/i18n/fold-for-match";
+
 /**
  * The catalogue, in display order. `key` is the stable machine value that
  * crosses the API and the Coach prompt; `labelKey` is the i18n key the chip
@@ -85,14 +87,7 @@ export const GLP1_SIDE_EFFECT_TAG_LABEL_KEYS: Record<
  * the label the matcher indexes.
  */
 export function normaliseSideEffectTag(raw: string): string {
-  return raw
-    .toLowerCase()
-    .normalize("NFD")
-    .replace(/[\u0300-\u036f]/g, "")
-    .replace(/[\u2018\u2019\u02bc'`]/g, " ")
-    .replace(/[-_/]/g, " ")
-    .replace(/\s+/g, " ")
-    .trim();
+  return foldForMatch(raw);
 }
 
 /**