prolog: cut !/0 with two-cut-box barrier scheme, 6 tests

plans/prolog-on-sx.md

@@ -52,7 +52,7 @@ Representation choices (finalise in phase 1, document here):
 ### Phase 3 — clause DB + DFS solver + cut + first classic programs
 - [x] Clause DB: `"functor/arity" → list-of-clauses`, loader inserts — `pl-mk-db` / `pl-db-add!` / `pl-db-load!` / `pl-db-lookup` / `pl-db-lookup-goal`, 14 tests in `tests/clausedb.sx`
 - [x] Solver: DFS with choice points backed by delimited continuations (`lib/callcc.sx`). On goal entry, capture; per matching clause, unify head + recurse body; on failure, undo trail, try next — first cut: trail-based undo + CPS k (no shift/reset yet, per briefing gotcha). Built-ins so far: `true/0`, `fail/0`, `=/2`, `,/2`. Refactor to delimited conts later.
-- [ ] Cut (`!`): cut barrier at current choice-point frame; collapse all up to barrier
+- [x] Cut (`!`): cut barrier at current choice-point frame; collapse all up to barrier — two-cut-box scheme: each `pl-solve-user!` creates a fresh inner-cut-box (set by `!` in this predicate's body) AND snapshots the outer-cut-box state on entry. After body fails, abandon clause alternatives if (a) inner was set or (b) outer transitioned false→true during this call. Lets post-cut goals backtrack normally while blocking pre-cut alternatives. 6 cut tests cover bare cut, clause-commit, choice-commit, cut+fail, post-cut backtracking, nested-cut isolation.
 - [ ] Built-ins: `=/2`, `\\=/2`, `true/0`, `fail/0`, `!/0`, `,/2`, `;/2`, `->/2` inside `;`, `call/1`, `write/1`, `nl/0`
 - [ ] Arithmetic `is/2` with `+ - * / mod abs`
 - [ ] Classic programs in `lib/prolog/tests/programs/`:
@@ -88,6 +88,7 @@ Representation choices (finalise in phase 1, document here):
 
 _Newest first. Agent appends on every commit._
 
+- 2026-04-25 — Cut (`!/0`) landed. `pl-cut?` predicate; solver functions all take a `cut-box`; `pl-solve-user!` creates a fresh inner-cut-box and snapshots `outer-was-cut`; `pl-try-clauses!` abandons alternatives when inner.cut OR (outer.cut transitioned false→true during this call). 6 new cut tests in `tests/solve.sx` covering bare cut, clause-commit, choice-commit, cut+fail blocks alt clauses, post-cut goal backtracks freely, inner cut isolation. Total 110 (+6).
 - 2026-04-25 — Phase 3 DFS solver landed (CPS, trail-based backtracking; delimited conts deferred). `pl-solve!` + `pl-solve-eq!` + `pl-solve-user!` + `pl-try-clauses!` + `pl-solve-once!` + `pl-solve-count!` in runtime.sx. Built-ins: `true/0`, `fail/0`, `=/2`, `,/2`. New `tests/solve.sx` 18/18 green covers atomic goals, =, conjunction, fact lookup, multi-solution count, recursive ancestor rule, trail-undo verification. Bug fix: `pl-instantiate` had no `("clause" h b)` case → vars in rule head/body were never instantiated, so rule resolution silently failed against runtime-var goals. Added clause case to recurse with shared var-env. Total 104 (+18).
 - 2026-04-24 — Phase 3 clause DB landed: `pl-mk-db` + `pl-head-key` / `pl-clause-key` / `pl-goal-key` + `pl-db-add!` / `pl-db-load!` / `pl-db-lookup` / `pl-db-lookup-goal` in runtime.sx. New `tests/clausedb.sx` 14/14 green. Total 86 (+14). Loader preserves declaration order (append!).
 - 2026-04-24 — Verified phase 1+2 already implemented on loops/prolog: `pl-parse-tests-run!` 25/25, `pl-unify-tests-run!` 47/47 (72 total). Ticked phase 1+2 boxes.