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/flows.py

@@ -837,7 +837,7 @@ def read_message_history(
     """The recent values of one message, for plotting.
 
     ``message`` may be given bare or qualified; a message that never carried a
-    number comes back with an empty series.
+    number or a flag comes back with an empty series.
     """
     _read_flow(controller, name)
     key = qualify(name, message)