feedback7 R22-HUD: a renderer that loses the 12 s race no longer mounts its canvas late — the attempt is aborted, a view that still comes back is destroyed, the host is no longer wiped (PR #302 by @siruimei07); golden: unchanged

Applied unchanged after review (Grok static review: MERGE; the diff read in full: no network, no new dependency, no
shipped eval). The leak was real on feedback7: mountFieldView raced createFieldView against a timeout and, on failure,
removed every child of the host while the factory kept running — Pixi finished later and appended its canvas over the
simplified view (taking its clicks); the host wipe could also delete a newer view mounted by the same hook.

- ui/fieldHost.js: mountFieldView(host, { signal }) owns an AbortController; the timeout, a failed import or an invalid
  view aborts it, a view a factory returns after that is destroyed, an outer abort (useFieldView's cleanup) rejects
  without a fallback; nothing clears the host's children any more.
- render/app.js createFieldView takes the signal: the Pixi load / manifest / font waits end on abort, the synchronous
  build registers its disposables (setupCleanup) so a throw releases the app, tiles, fx, impostors, listeners and
  observers it made, the board-art and 3D-board waits moved after the view exists (aborting them is view.destroy(),
  idempotent), destroy() also removes the canvas. render/app/host.js withTimeout(p, ms, signal).
- Tests: test/ui/fieldHost-lifecycle.test.js (5, node vm, fake timers: all fail on the parent) and the opt-in real-browser
  test/render/initialization.browser.test.js (6 scenarios in headless Chrome, 5 fail on the parent: a late Pixi load
  covering the fallback — with a factory that honours the signal and one that ignores it —, art timeout, a setup
  exception, an unmount racing a newer mount); test/ui/playtest2.test.js's source pin takes the new indentation.
  fieldview / browser suites in Chrome unchanged (the two 4× throttled fps tests flake when two Chrome files run
  at once, pass alone).
- The createFieldView body stays unindented inside its new try (the PR's choice, to keep the diff reviewable).
- Docs: design/client.md §9. Golden: unchanged (client lifecycle only).

public/js/render/app/host.js

@@ -4,8 +4,22 @@ import { createAssets, assets as defaultAssets } from '../../assets.js';
 import { standInRecord } from '../../../../shared/standIn.js';
 import { diyRecordOf } from '../../../../shared/diy.js';
 
-function withTimeout(p, ms) {
-  return Promise.race([p, new Promise((resolve) => setTimeout(resolve, ms))]);
+/** Wait for optional assets, or stop waiting when this view is cancelled. Shared loads keep running. */
+function withTimeout(p, ms, signal) {
+  let timer;
+  let onAbort;
+  const stopped = new Promise((resolve, reject) => {
+    if (ms != null) timer = setTimeout(resolve, ms);
+    if (signal) {
+      onAbort = () => reject(signal.reason);
+      signal.addEventListener('abort', onAbort, { once: true });
+      if (signal.aborted) onAbort();
+    }
+  });
+  return Promise.race([p, stopped]).finally(() => {
+    clearTimeout(timer);
+    if (onAbort) signal.removeEventListener('abort', onAbort);
+  });
 }
 
 /** Wrap whatever the caller passed as `assets` into the store API of public/js/assets.js. */