[dbrattli]

Why

flatMapActorSupervised shares a monitor mailbox between upstream values and child-exit signals. On BEAM, Fable.Actor's structural exit check can misread a source record with Pid and Reason fields as an exit. An actual ChildExited source value collides on every target. Python subscription disposal also failed because a partially applied cleanup function compiled to a callable that still required an argument.

How

• Wrap forwarded values in a private DU and classify the envelope before child exits. Unexpected monitor messages now surface as errors.
• Split output delivery, monitor routing, child-exit decisions, and cleanup into focused helpers.
• Pass an explicit zero-argument cleanup closure so Python disposal runs.
• Add shared regressions for exit-shaped records, ChildExited values, mixed payloads, ordering, and disposal. Refresh the measured runtime counts.

Closes #27