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).

(cherry picked from commit f70d3fda)

Ported onto main (#1717 review): the collision message is filed under
the file group it is looked up in (it sat under view), the data-link
warning reuses that cataloged text, and the command-reference string
that rode along in the locale hunks stays out.

locales/ru-RU/opencadstudio.ftl

@@ -2398,6 +2398,7 @@ command =
     .tcount-placement-overwrite-prefix-suffix-overw = TCOUNT  Размещение [Заменить/Префикс/Суффикс] <Заменить>:
 
 file =
+    .already-loaded-different-contents-kept = "__ocs_fmt_0__" уже загружен с другим содержимым; существующий файл сохранён.
     .parse-ms-purge-ms-caches-ms-xref-ms-total-ms = parse __ocs_fmt_0__ms · чистка __ocs_fmt_1__ms · кэши __ocs_fmt_2__ms · xref __ocs_fmt_3__ms · общая __ocs_fmt_4__ms
     .archive-save-the-drawing-first-it-has-no-file = ARCHIVE: сначала сохраните чертёж (путь к файлу ещё не задан).
     .autosaved-1-drawing = Автоматически сохранён 1 чертёж