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.

lib/minikanren/tests/nafc.sx

@@ -0,0 +1,50 @@
+;; lib/minikanren/tests/nafc.sx — Phase 5 piece C tests for `nafc`.
+
+(mk-test
+  "nafc-failed-goal-succeeds"
+  (run* q (nafc (== 1 2)))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "nafc-successful-goal-fails"
+  (run* q (nafc (== 1 1)))
+  (list))
+
+(mk-test
+  "nafc-double-negation"
+  (run* q (nafc (nafc (== 1 1))))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "nafc-with-conde-no-clauses-succeed"
+  (run*
+    q
+    (nafc
+      (conde ((== 1 2)) ((== 3 4)))))
+  (list (make-symbol "_.0")))
+
+(mk-test
+  "nafc-with-conde-some-clause-succeeds-fails"
+  (run*
+    q
+    (nafc
+      (conde ((== 1 1)) ((== 3 4)))))
+  (list))
+
+;; --- composing nafc with == as a guard ---
+
+(mk-test
+  "nafc-as-guard"
+  (run*
+    q
+    (fresh (x) (== x 5) (nafc (== x 99)) (== q x)))
+  (list 5))
+
+(mk-test
+  "nafc-guard-blocking"
+  (run*
+    q
+    (fresh (x) (== x 5) (nafc (== x 5)) (== q x)))
+  (list))
+
+(mk-tests-run!)