List traversal

list.tri is a native module with an explicit recursion_support dependency providing pair and the normalization-safe y. Preparation/check commands live in docs/build-and-run.md; the formatter is its first consumer.

• listFoldUntil input initial step: step head state returns a decision.
• listFoldTailUntil input initial step: step head tail state also receives the original tail, not a folded or reconstructed result.
• Both return pair finalState remainingInput.
• listContinue next consumes the current cell and updates state.
• listStop final updates state but stops before consuming that cell. The returned suffix includes it. Exhaustion returns an empty suffix.

Only exact t t continues and exact t commits a stopping state. Malformed callback decisions stop without committing their payload; malformed list tails are returned untouched. A nonempty suffix therefore means traversal halted, not successful exhaustion. These helpers do not validate callback behavior.

The worker inspects input before state/control, calls no callback on empty or non-cons input, and never visits subsequent cells after a stop. Selected callback work can still diverge. The fixed point and decision dispatch are local patterned groups inside the public wrapper; callers capture immutable configuration in their own local step groups rather than threading it through recursive worker calls.