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/lib/compareChartData.ts

@@ -10,6 +10,17 @@
 
 import type { ComparisonData } from './types';
 
+/**
+ * A yearly row only counts once it carries a real academic year. A school with
+ * no performance data still comes back from /api/compare with a single phantom
+ * row (the dim_school LEFT JOIN) where `year` is null — and Math.trunc(null) is
+ * 0, which would seed the axis at year 0 and, via fillAcademicYears, blow past
+ * every real year and blank all schools' lines. Drop those rows up front.
+ */
+function hasYear(row: { year: number }): boolean {
+  return typeof row.year === 'number' && Number.isFinite(row.year);
+}
+
 /** 201819 → 201920 (academic-year arithmetic on YYYYYY codes). */
 function nextAcademicYear(year: number): number {
   const start = Math.floor(year / 100);
@@ -66,14 +77,19 @@ export function buildCompareChart(
   nationalByYear?: Record<number, number | null | undefined>,
 ): CompareChart {
   const rawYears = schools.flatMap(
-    (s) => comparisonData[String(s.urn)]?.yearly_data.map((d) => Math.trunc(d.year)) ?? [],
+    (s) =>
+      comparisonData[String(s.urn)]?.yearly_data.filter(hasYear).map((d) => Math.trunc(d.year)) ??
+      [],
   );
   const years = fillAcademicYears(rawYears);
 
   const schoolDatasets: CompareChartSeries[] = schools.map((school, schoolIndex) => {
     const rows = comparisonData[String(school.urn)]?.yearly_data ?? [];
     const byYear = new Map<number, Record<string, unknown>>();
-    for (const row of rows) byYear.set(Math.trunc(row.year), row as unknown as Record<string, unknown>);
+    for (const row of rows) {
+      if (!hasYear(row)) continue;
+      byYear.set(Math.trunc(row.year), row as unknown as Record<string, unknown>);
+    }
     return {
       label: school.school_name,
       data: years.map((year) => {