Record flags in history and draw them as steps

A bool was excluded from the history as "not a measurement", so a true/false
port had no curve in the node panel and none on an edge — only the word. It is
recorded as 0/1 now and drawn as steps, since a bezier through two states
slopes through readings that never happened. The axis is pinned to 0..1, so a
flag that was never on sits at the floor rather than mid-box.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/api/routes/messages.py

@@ -102,7 +102,7 @@ async def publish_message(
 
 @router.get("/{name}/history", response_model=MessagePoints)
 def read_message_history(name: str, controller: FlowControllerDep) -> Any:
-    """The series behind a chart. Numbers only — nothing else plots."""
+    """The series behind a chart. Numbers and flags — nothing else plots."""
     if controller.pipeline is None:
         return MessagePoints(message=name, numeric=False, points=[])
     series = controller.state.history(name)