copy(web): replace em dashes in public copy

Rewrites every visitor-facing string that used an em dash, choosing the
punctuation for what the dash was doing: a colon before a list or
explanation, a comma for an aside, a full stop between two thoughts,
parentheses for an aside mid-sentence. Covers page titles and meta
descriptions, the home and admissions guide copy, school page headings
and notes, the compare page, metric labels and tooltips.

Two rewrites also fix the sentence around them: the closure banner no
longer repeats "proposed for closure", and the cut-off caveat's list of
priorities now parses.

A lone dash marking a missing value in a table cell stays: it is a data
convention, not prose. A Jest guard walks the source with the TypeScript
parser and fails on any other em dash in a string or JSX text node.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

nextjs-app/lib/utils.ts

@@ -116,7 +116,7 @@ export function formatWithSuppression(value: number | null | undefined): { displ
   return { display: formatPercentage(value), suppressed: false };
 }
 
-export const SUPPRESSED_TOOLTIP = 'Data not available — may be suppressed to protect small cohorts.';
+export const SUPPRESSED_TOOLTIP = 'Data not available. It may be suppressed to protect small cohorts.';
 
 /**
  * Format a progress score (can be negative)
@@ -750,7 +750,7 @@ export function buildSchoolSummary(
     if (admissions.first_preference_offer_pct != null) {
       const pct = Math.round(admissions.first_preference_offer_pct);
       parts.push(
-        `oversubscribed — ${pct}% of first-choice applicants are offered a place`,
+        `oversubscribed (${pct}% of first-choice applicants are offered a place)`,
       );
     } else {
       parts.push('oversubscribed');