ux: block with a "finding xrefs…" overlay while gathering xrefs

Gathering xrefs (xrefs_to + function_of over every site) runs in a worker, so the
UI stayed live and a second 'x' (or other action) fired into a half-finished
operation — duplicate requests, dialogs stacking, generally confusing behavior.

Add a small blocking BusyScreen modal shown the instant 'x' is pressed:
* re-entry guard (_xref_active) ignores a second 'x' while one gather is in
  flight, and the modal captures input anyway;
* _present_xrefs dismisses it and opens the dialog when results are ready;
* Esc cancels — a worker that finishes after the cancel won't pop a stale dialog
  (the _xref_active flag gates it);
* every exit path clears it (no-subject case, and a try/except around the
  worker's slow calls) so it can't get stuck.

Verified: 1st 'x' -> BusyScreen (xref_active=True); 2nd 'x' ignored (stack
unchanged); results -> dialog + busy gone; Esc -> cancelled, no late dialog.
follow_xrefs/xref_labels/decomp_nav/decomp_follow_self/view_toggle/disasm_nav/
search/mouse/listing_view/continuous_view/func_banners/startup green.

0 files changed, 0 insertions, 0 deletions