langgraph: make is_control_flow_signal public

[Nik7A]

A host instrumenting its own LangGraph nodes has to separate a node that parked on a control-flow signal from one that crashed. Get it wrong and you sign a false failure — a record claiming a node failed when it merely stopped at a human gate and will re-execute and succeed. This library treats that as exactly as dishonest as a false success, so the predicate is load-bearing for record honesty, not a convenience.

It shipped underscore-prefixed and out of __all__, which left a host two options: import a private name, or restate the boundary itself.

The private import was the right call

bosun imports _is_control_flow_signal (src/bosun/core/audit.py) rather than hand-rolling. Verified against the installed runtime, GraphBubbleUp has four subclasses:

GraphBubbleUp
  GraphDrained
  GraphInterrupt
    NodeInterrupt
  ParentCommand

A hand-rolled isinstance(exc, (GraphInterrupt, ParentCommand)) misses GraphDrained today — and would drift silently the next time the runtime widens the boundary, quietly resuming false-failure signing with nothing to notice. A moved private symbol at least fails loudly at import. The predicate delegates to GraphBubbleUp instead of restating it, so it cannot drift. The only thing wrong with it was the underscore.

What changes

• is_control_flow_signal is public and in __all__.
• _is_control_flow_signal stays as an alias. Hosts importing it pin >=0.2,<0.3; removing it inside that range would break their resolve. Drop it in 0.3.
• The guard enumerates GraphBubbleUp's subclasses from the runtime's class tree rather than listing them — a list in the test would be the same brittle restatement the predicate exists to avoid.

No behaviour change: same function, same callers, new name.

Reported from the bosun migration, 2026-07-17.