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).

src/app/update/mod.rs

@@ -572,6 +572,16 @@ impl OpenCADStudio {
                         .file_name()
                         .map(|name| name.to_string_lossy().into_owned())
                         .unwrap_or_else(|| path.to_string_lossy().into_owned());
+                    // The start page stays clickable under the open overlay,
+                    // so a recent can race an in-flight open and clobber it
+                    // (its WebFileOpened then fails the id check and the
+                    // bytes are lost) — refuse like on_open_file does.
+                    if self.opening.is_some() {
+                        self.command_line.push_info(
+                            crate::t!("An open is already in progress.").as_ref(),
+                        );
+                        return Task::none();
+                    }
                     let state = std::sync::Arc::new(crate::io::OpenProgressState::new(
                         crate::app::OPEN_PHASE_READING,
                     ));