fix: final pre-PR review sweep

The final holistic + quality audit (three reviewers) returned GO with
four majors; all fixed.

- Start-page recents were the last unguarded writer of the open slot
  on web: clicking one under the open overlay clobbered the in-flight
  open and silently lost its bytes. Refused with the same in-progress
  notice the picker uses.
- The web quit-after-save continuation stalled: a Quit pending now
  survives the tab-save AND continues — offering to save the next
  dirty tab or finishing the quit, mirroring native.
- Data-extraction's multi-file pick is capped (16): every picked file
  becomes a full parsed document held for the session, and one giant
  pick could exhaust the wasm heap.
- The xref registry tests raced each other on the process-global
  registry (reproduced 3/10 at high thread counts — CI roulette); they
  now hold a module-level guard.

Minors: the collision warning is cataloged in all 21 locales; the
docs' parallelism detail header no longer contradicts its own table;
the drop-filter and name-hash helpers gained inline tests (their only
prior coverage was a verbatim copy in the red-team suites).

locales/ru-RU/opencadstudio.ftl

@@ -3573,6 +3573,7 @@ view =
     .opening-ocs-discussions-for-help-and-questions = Открытие OCS Дискуссии о помощи и вопросах
     .opening-ocs-web = Открытие OCS Веб-сайт
     .opening-the-command-reference = Открытие справочника команд...
+    .already-loaded-different-contents-kept = "__ocs_fmt_0__" уже загружен с другим содержимым; существующий файл сохранён.
     .opening-patreon-page = Открытие страницы Patreon...
     .opening-feedback-page = Открываем страницу обратной связи...
     .opening-release-notes = Открытие выпускных заметок...