fix(compare): a no-results school no longer blanks the trend chart

Adding a school with no performance data to a comparison made every
school's trend line disappear until that school was removed.

Root cause: /api/compare returns such a school with a single phantom
yearly_data row (the dim_school LEFT JOIN) whose year is null. In
buildCompareChart, Math.trunc(null) is 0, so the axis was seeded at
year 0; fillAcademicYears then walked 0, 101, 202, … and hit its
50-step cap long before reaching the real years, leaving every
school's series mapped entirely to null.

Fix: ignore yearly rows without a real numeric year when building the
axis and the per-school year map. Regression test added.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nextjs-app/__tests__/lib/compareChartData.test.ts

@@ -41,6 +41,30 @@ describe('buildCompareChart', () => {
     }
   });
 
+  it('ignores a no-results school whose only row has a null year', () => {
+    // A school with no performance rows comes back from /api/compare with a
+    // single phantom yearly_data row (LEFT JOIN) where year and every metric
+    // are null. That null year must NOT pollute the axis: Math.trunc(null) is
+    // 0, and filling from year 0 blows past the real years, blanking every
+    // school's line. Regression guard for "add a no-data school → chart empty".
+    const withNoData = {
+      ...THREE_SCHOOLS,
+      '4': school(4, [[null as unknown as number, null]]),
+    };
+    const list = [...SCHOOL_LIST, { urn: 4, school_name: 'School 4' }];
+    const chart = buildCompareChart(withNoData, list, 'rwm_expected_pct');
+
+    // The real years still drive the axis; the phantom year 0 is gone.
+    expect(chart.years).toContain(201819);
+    expect(chart.years).toContain(202425);
+    expect(chart.years).not.toContain(0);
+    // The three real schools still render their lines.
+    for (const urn of [1, 2, 3]) {
+      const ds = chart.schoolDatasets[urn - 1];
+      expect(ds.data.some((v) => v != null)).toBe(true);
+    }
+  });
+
   it('handles float years from the API (202425.0 style)', () => {
     const floaty = {
       '1': school(1, [[201819.0 as number, 80], [202425.0 as number, 85]]),