Stop the strict acceptance from waiting for itself

A scenario whose body ran past about 19 s idled until 45 s after its
start, and twice one never exited and hit the runner's 300 s timeout
after it had passed: something of Chrome outlived Browser.close and kept
its stdout and stderr open.

- scripts/qa/browser.mjs closes Chrome within 5 s, kills its process
  group and closes its pipes; the harness then finishes and exits with
  the scenario's code. QA_EXIT_DIAGNOSTICS=1 keeps the old ending and
  prints the open handles and Chrome's process tree 5 s after finish.
- Every strict page starts with its clock at the 18:30 UTC nearest to
  the real time, so Navara's sun is up over the test cities whenever the
  acceptance runs; a scenario can ask for its own instant or the real
  clock (Wildlife does, for its freshness fixtures).
- The runner records a scenario that passed but did not exit as hung,
  kills the whole process group on a timeout, runs a failed scenario
  once more and reports one that passes then as flaky, and appends each
  run to --history=FILE; scripts/report-acceptance-flake.mjs reads it.

scripts/qa-app-once.mjs

@@ -1,3 +1,4 @@
+import { closeBrowser, exitAfterFlush } from './qa/browser.mjs';
 import { localPreview, waitForEngine } from './qa-strict-harness.mjs';
 // The application is created once per page: switching engines detaches and
 // attaches only the engine-bound parts. This gate runs three Cesium→Navara→
@@ -413,6 +414,10 @@ try {
 
   console.log(JSON.stringify(report, null, 2));
   if (!reportOnly) console.log('PASS application created once; session kept');
+} catch (error) {
+  console.error(error);
+  process.exitCode = 1;
 } finally {
-  await browser.close();
+  await closeBrowser(browser);
 }
+await exitAfterFlush();