fix(review): don't suppress a genuine mainstream 0 in the rows

Review feedback: the rankings/search rows treated any exactly-0 attainment as
not comparable regardless of school type, so a genuine 0 at a mainstream
school rendered as "—" (looks like missing data) and dropped its delta —
stricter and inconsistent with SchoolDetailView's placeholder check.

- SchoolRow: match SchoolDetailView's ks2Placeholder signature — suppress only
  when ALL of RWM + reading + writing + maths are 0 (special/suppressed
  signature), not on a bare rwm === 0. A genuine 0% combined (some pupils met
  individual subjects but not all three) is not all-zero, so it stays
  comparable and shows its real figure + trend.
- SecondarySchoolRow: Attainment 8 is a single 0–80 score with no subject
  breakdown to form an all-zero signature, so key off establishment type only
  (drop the bare att8 === 0 guard). A genuine (if extreme) 0.0 shows its value
  + LA delta.
- For consistency, apply the same to the detail views: drop the bare
  attainment_8_score === 0 guard (KS4 keys off isSpecial only); KS2 keeps the
  all-four-subjects-zero placeholder signature.

Special schools / PRUs / AP are still handled via isSpecialSchool everywhere.
tsc clean; 108/108 unit.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

nextjs-app/components/SecondarySchoolRow.tsx

@@ -57,8 +57,11 @@ export function SecondarySchoolRow({
   const att8 = school.attainment_8_score;
   // Special schools / PRUs / AP: Attainment 8 vs the LA average isn't a fair
   // comparison (their pupils have SEND), so drop the delta and the number
-  // rather than show them trailing "the average" by design.
-  const att8Comparable = att8 != null && att8 !== 0 && !isSpecialSchool(school);
+  // rather than show them trailing "the average" by design. Attainment 8 is a
+  // single 0–80 score with no subject breakdown to test for a placeholder, so
+  // this keys off establishment type only — a genuine (if extreme) 0.0 at a
+  // mainstream school still shows its real value.
+  const att8Comparable = att8 != null && !isSpecialSchool(school);
   const laDelta =
     att8Comparable && laAvgAttainment8 != null ? (att8 as number) - laAvgAttainment8 : null;