mk: phase 5C — nafc, negation as finite failure

(nafc g) is a three-line primitive: peek the goal's stream for one
answer; if empty, yield (unit s); else mzero. Carries the standard
miniKanren caveats — open-world unsound, diverges on infinite streams.

7 tests: failed-goal-succeeds, successful-goal-fails, double-negation,
conde-all-fail-makes-nafc-succeed, conde-any-success-makes-nafc-fail,
nafc as a guard accepting and blocking.

201/201 cumulative.

plans/minikanren-on-sx.md

@@ -128,7 +128,9 @@ Key semantic mappings:
 - [x] `conda` — soft-cut: first non-failing head wins; ALL of head's answers
       flow through rest-goals; later clauses not tried (`Phase 5 piece A`)
 - [x] `condu` — committed choice (Phase 2)
-- [ ] `nafc` — negation as finite failure with constraint
+- [x] `nafc` — negation as finite failure: `(nafc g)` yields the input subst
+      iff g has zero answers. Standard caveats apply (open-world unsoundness;
+      diverges if g is infinite). `Phase 5 piece C`.
 - [ ] Tests: Zebra puzzle, N-queens, Sudoku via `project`, family relations via `matche`
 
 ### Phase 6 — arithmetic constraints CLP(FD)
@@ -157,6 +159,9 @@ _(none yet)_
 
 _Newest first._
 
+- **2026-05-07** — **Phase 5 piece C — nafc**: `lib/minikanren/nafc.sx`. Three-line
+  primitive: stream-take 1; if empty, `(unit s)`, else `mzero`. 7 tests including
+  double-negation and use as a guard. 201/201 cumulative.
 - **2026-05-07** — **Phase 5 piece B — project**: `lib/minikanren/project.sx` —
   defmacro that walks each named var, rebinds them, and runs the body's mk-conj.
   Demonstrated escape into host arithmetic / string ops (`(* n n)`, `(str s "!")`).