mk: phase 2B — fresh, defmacro form + call-fresh

(fresh (x y z) g1 g2 ...) expands to a let that calls (make-var) for each
named var, then mk-conjs the goals. call-fresh is the function-shaped
alternative for programmatic goal building.

9 new tests: empty-vars, single var, multi-var multi-goal, fresh under
disj, nested fresh, call-fresh equivalents. 91/91 cumulative.

lib/minikanren/fresh.sx

@@ -0,0 +1,23 @@
+;; lib/minikanren/fresh.sx — Phase 2 piece B: `fresh` for introducing
+;; logic variables inside a goal body.
+;;
+;;   (fresh (x y z) goal1 goal2 ...)
+;;     ≡ (let ((x (make-var)) (y (make-var)) (z (make-var)))
+;;         (mk-conj goal1 goal2 ...))
+;;
+;; A macro rather than a function so user-named vars are real lexical
+;; bindings — which is also what miniKanren convention expects.
+;; The empty-vars form (fresh () goal ...) is just a goal grouping.
+
+(defmacro
+  fresh
+  (vars &rest goals)
+  (quasiquote
+    (let
+      (unquote (map (fn (v) (list v (list (quote make-var)))) vars))
+      (mk-conj (splice-unquote goals)))))
+
+;; call-fresh — functional alternative for code that builds goals
+;; programmatically:
+;;   ((call-fresh (fn (x) (== x 7))) empty-s) → ({:_.N 7})
+(define call-fresh (fn (f) (fn (s) ((f (make-var)) s))))