Fix #8: scheduler survives link death; timed-out one-offs cancelled

- A transport exception in the poll loop killed the thread silently, leaving the
  GUI on a frozen 'Connected' dashboard and blocking run_oneoff callers for the
  full timeout. _loop now catches it -> stops, fails pending one-offs with the
  real error, and calls an on_error callback. Controller wires on_error to flag
  the connection dead; the GUI detects it in _tick and tears down with a
  'Connection lost' dialog.
- A run_oneoff that timed out left its job queued, so it executed LATER on the
  shared link -- a ghost/duplicate vehicle command. Jobs now carry
  cancelled/started flags under a lock; on timeout a not-yet-started job is
  cancelled (skipped by _drain_oneoffs), and a started one reports 'still
  running -- do NOT retry'. stop() also frees stranded submitters.
- tests/test_scheduler.py: cancel-on-timeout, freed-on-death, loop-survives.

Closes #8

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016yT89n4zR4qbrySoSiEyZs

gui/main.py

@@ -775,6 +775,17 @@ class MainWindow(QtWidgets.QMainWindow):
                 b.setEnabled(False)
         self.status.showMessage("Disconnected.")
 
+    def _on_link_lost(self, exc):
+        """The polling thread died (transport failure). Tear down and tell the
+        user instead of leaving a frozen 'Connected' dashboard."""
+        self.ctl.poll_error = None                    # one-shot
+        self._disconnect()
+        self.status.showMessage(f"Connection lost: {exc}")
+        QtWidgets.QMessageBox.warning(
+            self, "Connection lost",
+            f"The adapter connection failed and polling stopped:\n\n{exc}\n\n"
+            "Check the adapter/cable and reconnect.")
+
     # ---------- PID selection ----------
     def _apply_preset(self, name):
         if not self.ctl.connected:
@@ -1001,6 +1012,8 @@ class MainWindow(QtWidgets.QMainWindow):
 
     def _tick(self):
         if not self.ctl.connected:
+            if getattr(self.ctl, "poll_error", None) is not None:
+                self._on_link_lost(self.ctl.poll_error)
             return
         self.tree.blockSignals(True)
         for key, it in self._items.items():