fix(dashboard): headline newest available run

tests/lib/dashboard/dashboard.test.mjs

@@ -8,7 +8,7 @@ import { rollupStatus, fmtDuration, fmtBytes, pct, decodeMungedName, esc, STATUS
 import { niceMax, bounds, lineChart, statusBar, sparkline } from './charts.mjs';
 import { isUnifiedReport, runIdFor, summarize, mediaCount, safeId } from './ingest.mjs';
 import { reportMetrics, scaleSweepMetrics, largeGraphMetrics, physicsMetrics, vlmMetrics, pointKey } from './metrics.mjs';
-import { mergeRuns, mergeMetrics, readJsonl, appendJsonl, snapshotRun, pruneSnapshots } from './history.mjs';
+import { mergeRuns, mergeMetrics, readJsonl, appendJsonl, snapshotRun, pruneSnapshots, pickLatest } from './history.mjs';
 
 // ── format ────────────────────────────────────────────────────────────────
 test('rollupStatus: worst-of precedence', () => {
@@ -186,6 +186,13 @@ test('mergeRuns tie-breaks equal finishedAt deterministically by runId', () => {
   const { merged } = mergeRuns([], [{ runId: 'b', finishedAt: 5 }, { runId: 'a', finishedAt: 5 }]);
   assert.deepEqual(merged.map((r) => r.runId), ['a', 'b']);
 });
+test('pickLatest prefers newest available run, skips re-stamped/unavailable headline', () => {
+  assert.equal(pickLatest([]), null);
+  assert.equal(pickLatest(null), null);
+  assert.equal(pickLatest([{ runId: 'r2', available: false }, { runId: 'r1', available: true }]), 'r1');
+  assert.equal(pickLatest([{ runId: 'r2', available: true }, { runId: 'r1', available: true }]), 'r2');
+  assert.equal(pickLatest([{ runId: 'r2', available: false }, { runId: 'r1', available: false }]), 'r2');
+});
 test('mergeMetrics dedupes by key', () => {
   const { merged, added } = mergeMetrics([{ key: 'a' }], [{ key: 'a' }, { key: 'b' }, { key: 'b' }]);
   assert.equal(merged.length, 2);

tests/lib/dashboard/history.mjs

@@ -24,6 +24,18 @@ export function mergeRuns(existing, incoming) {
   return { merged, added };
 }
 
+/**
+ * Pick the headline "latest" run for the dashboard. Runs come in newest-first
+ * (as mergeRuns sorts them). Prefer the newest run whose report is actually on
+ * disk (available) so the headline never points at a re-stamped/cached entry
+ * that 404s on drill-down; fall back to the newest run overall, else null.
+ */
+export function pickLatest(runs) {
+  if (!Array.isArray(runs) || !runs.length) return null;
+  const available = runs.find((r) => r && r.available);
+  return (available || runs[0]).runId;
+}
+
 export function mergeMetrics(existing, incoming) {
   const seen = new Set(existing.map((m) => m.key));
   const added = [];

tests/lib/dashboard/server.mjs

@@ -16,7 +16,7 @@ import { resolve, join, sep, extname } from 'node:path';
 import { spawn } from 'node:child_process';
 import { discover, signature, safeId, summarize } from './ingest.mjs';
 import { reportMetrics, scanPerfArtifacts } from './metrics.mjs';
-import { mergeRuns, mergeMetrics, readJsonl, appendJsonl, snapshotRun, pruneSnapshots } from './history.mjs';
+import { mergeRuns, mergeMetrics, readJsonl, appendJsonl, snapshotRun, pruneSnapshots, pickLatest } from './history.mjs';
 import { renderShell } from './page.mjs';
 
 const args = process.argv.slice(2);
@@ -104,7 +104,7 @@ function reindex() {
   const allMetrics = readJsonl(METRICS_JSONL);
   state = {
     generatedAt: Date.now(),
-    latest: merged[0] ? merged[0].runId : null,
+    latest: pickLatest(merged),
     runs: merged,
     metrics: allMetrics.slice(-6000),
     roots: ROOTS.map((r) => ({ name: r.name, label: r.label, mode: r.mode, exists: existsSync(r.path), count: discovered.filter((d) => d.source === r.name).length })),