test: find whichever Chromium build is installed

The parity runner pinned one build number, so a fresh
`playwright-core install chromium` left it pointing at a directory that
is not there. PARITY_CHROME still overrides.

tests/render-parity/run.mjs

@@ -11,7 +11,7 @@ import { chromium } from 'playwright-core';
 
 const HERE = dirname(fileURLToPath(import.meta.url));
 const ROOT = resolve(HERE, '../..');
-const CHROME = join(process.env.HOME, '.cache/ms-playwright/chromium-1228/chrome-linux/chrome');
+const CHROME = process.env.PARITY_CHROME ?? newestChromium();
 const PORT = 5199;
 const PAGE_GAP = 20;           // pageBreaks.ts
 const PT_MM = 25.4 / 72;
@@ -19,6 +19,17 @@ const PX_MM = 25.4 / 96;
 const LINE_TOL_MM = 1.2;       // words within this vertical span are one line
 const POS_TOL_MM = 1.0;        // reported as a position difference beyond this
 
+// `playwright-core install chromium` names the directory after its own build number,
+// so take the highest one present. PARITY_CHROME overrides it.
+function newestChromium() {
+  const base = join(process.env.HOME, '.cache/ms-playwright');
+  const builds = (existsSync(base) ? readdirSync(base) : [])
+    .filter((d) => /^chromium-\d+$/.test(d))
+    .sort((a, b) => +b.slice(9) - +a.slice(9));
+  if (!builds.length) throw new Error('no Chromium in ~/.cache/ms-playwright — see README.md');
+  return join(base, builds[0], 'chrome-linux/chrome');
+}
+
 // ---------------------------------------------------------------- reference
 
 function loRender(file, work) {