`src/engine/core/outcome.rs` — outcome → error

One small file with one load-bearing job: the canonical mapping from a terminal MachineOutcome to the LoopError a caller sees.

Key items

• MachineOutcome::to_loop_error(max_turns) -> Option<LoopError>:
  • Completed { .. } → None (a clean completion is not an error)
  • MaxTurnsExceeded → MaxTurnsExceeded { max }
  • Cancelled → Cancelled
  • Failed { error } → the error itself

Behavior notes

• Single source of truth: both the driver’s Done arm and Loop::stop_reason() go through it — the two can never disagree.
• The Done arm additionally guards the future: a non-Completed outcome that maps to no error becomes LoopError::Internal("unmapped terminal outcome: ...") rather than committing as success — adding a MachineOutcome variant without mapping it is a loud bug, not a silent one.

Deep dive: Termination.