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/components/SpecialSchoolNote.tsx

@@ -33,14 +33,14 @@ export function SpecialSchoolNote({ school }: { school: { school_type?: string |
         <>
           <strong>This is a special school.</strong> Its pupils have special educational needs and
           work towards individual targets. They sit the same national assessments, but very few
-          reach the mainstream &ldquo;expected standard&rdquo; these measures report — so a
+          reach the mainstream &ldquo;expected standard&rdquo; these measures report, so a
           comparison with the England average isn&rsquo;t a meaningful guide to the school.
         </>
       )}
       {kind === 'pru' && (
         <>
           <strong>This is a pupil referral unit.</strong> It teaches pupils educated outside a
-          mainstream school — for example after exclusion, or for medical or behavioural reasons.
+          mainstream school, for example after exclusion or for medical or behavioural reasons.
           The mainstream &ldquo;expected standard&rdquo; and the England-average comparison
           aren&rsquo;t a meaningful guide to the school.
         </>