Completion/escape refactor: EvalSignal retired

The evaluator’s result surface was reworked so that ordinary completion and non-local escape are represented by distinct types rather than one tagged signal. Three changes:

• Escape and BodyResult were introduced;
• the evaluator surface was flipped to Settled<Value>, with the tail call absorbed inside the mobile-state swap;
• EvalSignal together with its transitional shims was deleted.

This is the structural basis on which the escape-propagation bugs (escape-propagation-bugs) stay fixed: with completion and escape distinguished in the types, a try or grant boundary can no longer silently conflate a genuine escape with a recoverable result.

The migration is complete — both phases landed on 2026-05-14.

See also control-operators, core.