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.

src/locale_catalog.rs

@@ -2278,6 +2278,7 @@ pub(super) fn message_attribute(source: &str) -> Option<(&'static str, &'static
         "Opening \"{name}\" ({size_label})…" => Some(("common", "opening-name-size-label")),
         "Opening OCS Discussions for help and questions..." => Some(("view", "opening-ocs-discussions-for-help-and-questions")),
         "Opening OCS Web..." => Some(("view", "opening-ocs-web")),
+        "\"{name}\" is already loaded with different contents; the existing file was kept." => Some(("file", "already-loaded-different-contents-kept")),
         "Opening Patreon page..." => Some(("view", "opening-patreon-page")),
         "Opening feedback page..." => Some(("view", "opening-feedback-page")),
         "Opening file" => Some(("ui", "opening-file")),