Add real switch support for multi-device matrix radar

MultiDeviceLibreVnaService only exposes the 2x4 virtual combo matrix on
its own USB transport. When a physical GPIO switch sits on the master
stimulus and/or slave receiver path, the effective matrix is wider than
that. SwitchedMatrixRadarService wraps the inner service and drives the
extra switch(es) between acquire_collection calls, widening the combo
matrix by the physical position counts (matrix_output_switch_positions /
matrix_input_switch_positions in RunConfigModel).

Combo-matrix construction was centralized into
RunConfigModel.build_runtime_combos() so the GUI, workflows, and codec
all derive the same widened matrix instead of each computing its own
version of the virtual 2x4 layout.

python_app/hardware_full/switch_service.py

@@ -66,6 +66,10 @@ class SwitchService:
         """Switch to requested position."""
         self._driver.switch_to(position)
 
+    def position_count(self) -> int:
+        """Return number of positions supported by the backend driver."""
+        return self._driver.position_count()
+
     @property
     def current_position(self) -> int:
         """Return current switch position reported by backend driver."""