web: fix red-team breaks in restore, drop-open, and plot paths

Three attack agents (state-machine tracing, executable input fuzzing,
browser-API review) found two breaks and several degradations; all are
fixed here, with the fuzzer's 22 tests kept as regression suites.

Breaks:
- Untitled tabs (and dot-names like ".dxf") autosave as DXF, but the
  restore name defaulted to .dwg — the worker dispatches its reader by
  the name's extension, so the only copy of unsaved work could never
  be restored and was re-offered forever. recovered_name now uses the
  worker's rsplit('.') convention and defaults extensionless names to
  dxf, matching new-document serialization.
- " (recovered)" stacked one suffix per crash-restore cycle; the
  marker is now stripped idempotently before appending.
- Opening a file (Ctrl+O / OPEN / automation) while a web open was in
  flight clobbered it: the in-flight open's bytes were lost and every
  queued restore prompt stalled for the session. on_open_file now
  refuses while an open is running (web only).

Hardening:
- Escape on the Recover Drawing modal defers instead of deleting the
  only snapshot; the Discard button still removes it.
- A cap-rejected restore enqueue warns in-app and re-offers the next
  snapshot; the drop queue cap rises to 32.
- Exiting mid-round no longer races snapshot removals (deferrals are
  drained, the sweep skips while a round is in flight); closing a tab
  with "Don't save" now also removes its web snapshot; the snapshot
  key derivation is shared via web_autosave_key.
- The plot-preview overlay no longer leaks a full PDF copy per preview
  (the Download closure lives in a replaceable registry).
- A corrupt autosave index.json fails the round/boot instead of
  silently resetting every row; plot pages with non-finite or non-
  positive sheet dimensions error instead of emitting a NaN MediaBox.
- The drop filter accepts dot-less "DWG"/"dxf" names the worker
  parses (same rsplit convention on both gates).

(cherry picked from commit 3eab00bfbf)

Ported onto main (#1717 review): the five standalone tests/*_redteam.rs
files stay out — no test target builds them and they exercised copies of
the functions rather than the code itself.

src/app/update/dialog.rs

@@ -191,6 +191,22 @@ impl OpenCADStudio {
                 // Discarded — drop this tab's autosave recovery copy.
                 #[cfg(not(target_arch = "wasm32"))]
                 let _ = std::fs::remove_file(self.autosave_target(idx));
+                // Web counterpart: the OPFS snapshot must go too, or the
+                // drawing the user just declined to save is offered back at
+                // the next boot. A snapshot round still writing this tab's key
+                // would resurrect it after the remove, so defer to the round's
+                // end (the same guard the save flow uses).
+                #[cfg(target_arch = "wasm32")]
+                {
+                    let key = Self::web_autosave_key(&self.tabs[idx]);
+                    if self.web_autosave_in_flight {
+                        self.web_autosave_clear_after.push(key);
+                    } else {
+                        wasm_bindgen_futures::spawn_local(async move {
+                            let _ = crate::io::web_autosave::remove(&key).await;
+                        });
+                    }
+                }
                 if self.tabs.len() == 1 {
                     self.tab_counter += 1;
                     self.tabs[0] = crate::app::document::DocumentTab::new_drawing(self.tab_counter);