perf(wizard): parallelize Will-Executor broadcast in Building Will wizard

The Building Will wizard (BalBuildWillDialog.loop_push) still broadcast the
will to will-executors sequentially -- a for-loop calling
push_transactions_to_willexecutor one server at a time. This is the slow
"Broadcasting your will to executors: Trasmissione" step the user saw: a
slow/dead server blocked the whole wizard, just like the non-wizard path did
before it was parallelized.

Rewrite loop_push to use Willexecutors.push_transactions_parallel (the same
helper already used by window.push_transactions_to_willexecutors):
- Pre-filter to the user-selected will-executors only.
- Push to all selected servers concurrently (ThreadPoolExecutor); each server
  keeps its own retry behaviour, but a slow/dead server no longer blocks the
  others. Total time ~= slowest server, not the sum.
- on_each callback does thread-safe book-keeping + UI update via msg_edit_row
  (which emits a pyqtSignal marshalled to the GUI thread).
- 'already present' servers are collected and their stored tx verified
  sequentially afterwards (original check_transaction logic preserved).
- Preserve the retry flag and the _stopping cancellation checks.

tests/parallel_ping_test.py: add a static check asserting loop_push uses
push_transactions_parallel and no longer contains the sequential push loop.

Tests: 182 official + smoke/overflow/gui_fixes/parallel/external_zip all pass.
ruff: no new issues; new code is PEP8-compliant.

tests/parallel_ping_test.py

@@ -122,6 +122,25 @@ def main():
     finally:
         W.push_transactions_to_willexecutor = orig_push
 
+    # ---- 3) the wizard's loop_push must use the parallel helper ----
+    # The "Building Will" wizard broadcasts via BalBuildWillDialog.loop_push.
+    # It previously looped over servers sequentially (one
+    # push_transactions_to_willexecutor call at a time), which is exactly the
+    # slow path the user saw at "Broadcasting your will to executors".  Make
+    # sure it now delegates to push_transactions_parallel.
+    import inspect
+    dialogs_mod = importlib.import_module(f"{PKG}.gui.qt.dialogs")
+    loop_push_src = inspect.getsource(dialogs_mod.BalBuildWillDialog.loop_push)
+    code = "\n".join(
+        line for line in loop_push_src.splitlines()
+        if not line.lstrip().startswith("#")
+    )
+    assert "push_transactions_parallel" in code, (
+        "wizard loop_push must use push_transactions_parallel (parallel push)")
+    assert "for url, willexecutor in willexecutors.items()" not in code, (
+        "wizard loop_push must not push to servers in a sequential loop")
+    print("[OK] wizard loop_push uses push_transactions_parallel (not sequential)")
+
     print(f"\n[OK] parallel networking test passed for package {PKG!r}")
     return 0