fix: align perf history with perf-data branch layout

- Add history download step to unified pr-report.yaml reading from
  baselines/ on perf-data branch (the actual layout)
- Fix loadHistoricalReports to handle flat JSON files, not just
  subdirectories with perf-metrics.json
- Handle zero-baseline edge case in trendDirection: return 'rising'
  when firstMean=0 but secondMean>0 instead of masking as 'stable'

Addresses review feedback:
#9939 (review)

Author: christian-byrne

.github/workflows/pr-report.yaml

@@ -173,6 +173,19 @@ jobs:
           path: temp/perf-baseline/
           if_no_artifact_found: warn
 
+      - name: Download perf history from perf-data branch
+        if: steps.pr-meta.outputs.skip != 'true' && steps.find-perf.outputs.status == 'ready'
+        continue-on-error: true
+        run: |
+          if git ls-remote --exit-code origin perf-data >/dev/null 2>&1; then
+            git fetch origin perf-data --depth=1
+            mkdir -p temp/perf-history
+            for file in $(git ls-tree --name-only origin/perf-data baselines/ 2>/dev/null | sort -r | head -10); do
+              git show "origin/perf-data:${file}" > "temp/perf-history/$(basename "$file")" 2>/dev/null || true
+            done
+            echo "Loaded $(ls temp/perf-history/*.json 2>/dev/null | wc -l) historical baselines"
+          fi
+
       - name: Generate unified report
         if: steps.pr-meta.outputs.skip != 'true'
         run: >

scripts/perf-report.ts

@@ -70,8 +70,11 @@ function groupByName(
 function loadHistoricalReports(): PerfReport[] {
   if (!existsSync(HISTORY_DIR)) return []
   const reports: PerfReport[] = []
-  for (const dir of readdirSync(HISTORY_DIR)) {
-    const filePath = join(HISTORY_DIR, dir, 'perf-metrics.json')
+  for (const entry of readdirSync(HISTORY_DIR)) {
+    const entryPath = join(HISTORY_DIR, entry)
+    const filePath = entry.endsWith('.json')
+      ? entryPath
+      : join(entryPath, 'perf-metrics.json')
     if (!existsSync(filePath)) continue
     try {
       reports.push(JSON.parse(readFileSync(filePath, 'utf-8')) as PerfReport)

scripts/perf-stats.test.ts

@@ -182,6 +182,14 @@ describe('trendDirection', () => {
   it('returns stable for small fluctuations within 10%', () => {
     expect(trendDirection([100, 100, 100, 105, 105, 105])).toBe('stable')
   })
+
+  it('detects rising when baseline is zero but current is non-zero', () => {
+    expect(trendDirection([0, 0, 0, 5, 5, 5])).toBe('rising')
+  })
+
+  it('returns stable when both halves are zero', () => {
+    expect(trendDirection([0, 0, 0, 0, 0, 0])).toBe('stable')
+  })
 })
 
 describe('trendArrow', () => {

scripts/perf-stats.ts

@@ -93,7 +93,7 @@ export function trendDirection(values: number[]): TrendDirection {
   const firstMean = firstHalf.reduce((a, b) => a + b, 0) / firstHalf.length
   const secondMean = secondHalf.reduce((a, b) => a + b, 0) / secondHalf.length
 
-  if (firstMean === 0) return 'stable'
+  if (firstMean === 0) return secondMean > 0 ? 'rising' : 'stable'
   const changePct = ((secondMean - firstMean) / firstMean) * 100
 
   if (changePct > 10) return 'rising'