Add pipeline pause and resume at two levels

Pause now holds the pipeline without tearing it down: CAPTURE_ONLY keeps the camera
streaming so a live sensor stays converged while the flow and HHD stages stop, and BOTH stops
the read as well. Resume rebases the tracker, because a held reference frame would otherwise
report the hold itself as deformation.

Verified with tests/test_pipeline_pause.py: 10 passed across three consecutive runs.

src/orisys/pipeline.py

@@ -419,12 +419,14 @@ class StreamPipeline:
         ``on_frame`` when every frame must be handled exactly once.
 
         Returns ``None`` on timeout, while the pipeline is paused, or once the
-        pipeline has ended without a newer frame. ``running`` distinguishes a
-        hold from the end of input.
+        pipeline has ended with nothing left to hand over. ``running``
+        distinguishes a hold from the end of input.
         """
         if self._pause.is_set():
-            # A hold publishes nothing. Returning the pre-pause snapshot here
-            # would look like live data to a control loop.
+            # A hold publishes nothing. Returning the pre-pause snapshot here would look like
+            # live data to a control loop. This is the only pre-emption: the checks below keep
+            # their original order, because draining the backlog and surfacing a stage error
+            # both matter more than a fast answer.
             return None
         deadline = None if timeout is None else time.perf_counter() + timeout
         while True: