fix: bot-review robustness fixes (TTS sync, updater, nightly, a11y) (#4659)

Cherry-picked and re-verified the applicable subset of
julianshen/readest@fa1b74a0 (its "address PR #15 bot reviews" commit). The
fork-only AI-annotation-tool change was dropped — readest has no 'ai' toolbar
tool. Each logic fix is covered by a failing-first test.

- TTS position sequence is now an app-wide monotonic counter, so a fresh
  TTSController (constructed per `tts-speak`) isn't dropped by consumers holding
  `lastSequenceSeen` from a prior session.
- share.ts only swallows AbortError (user cancel); other failures — e.g.
  NotAllowedError when a quick action fires without a user gesture — fall back to
  the clipboard so the text still reaches the user.
- document.isTxt tolerates MIME params (text/plain;charset=utf-8), uppercase
  extensions (BOOK.TXT), and a nameless Blob, so a TXT can't slip onto the
  non-text path and yield a null book.
- updater getNightlyPlatformKey matches x86_64/aarch64 explicitly; a 32-bit or
  otherwise unknown arch yields no nightly instead of mis-routing to aarch64.
- UpdaterWindow downloadWithProgress resolves on tauriDownload completion even
  when Content-Length is absent (no more hang on portable/AppImage/Android).
- nightly_update.rs uses async tokio::fs::read in the async command.
- nightly.yml: serialize runs via a concurrency group (no cancel) and
  persist-credentials:false on checkouts.
- edge TTS route only emits the word-boundary header when it fits under ~8KB;
  oversized values get dropped by proxies, and the client falls back to [].
- RSVPOverlay drops the contradictory aria-disabled on the functional rate
  button (it opens the pace picker).
- nightly verify harness handles artifact stream errors instead of crashing.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/readest-app/scripts/nightly-verify-harness/serve.mjs

@@ -89,9 +89,17 @@ const handleRequest = (req, res) => {
       return json(res, buildStableManifest(false));
     case '/releases/latest-surpass.json':
       return json(res, buildStableManifest(true));
-    case '/artifacts/test.bin':
+    case '/artifacts/test.bin': {
+      const stream = fs.createReadStream(ARTIFACT);
+      // Handle a missing/unreadable artifact instead of crashing the harness on
+      // an unhandled stream 'error'.
+      stream.on('error', () => {
+        if (!res.headersSent) res.writeHead(500);
+        res.end('artifact error');
+      });
       res.writeHead(200, { 'content-type': 'application/octet-stream' });
-      return fs.createReadStream(ARTIFACT).pipe(res);
+      return stream.pipe(res);
+    }
     default:
       res.writeHead(404);
       return res.end('not found');