mk: phase 2A — streams + ==/conj/disj, 34 new tests

lib/minikanren/stream.sx: mzero/unit/mk-mplus/mk-bind/stream-take. Three
stream shapes (empty, mature list, immature thunk). mk-mplus suspends and
swaps on a paused-left for fair interleaving (Reasoned Schemer style).

lib/minikanren/goals.sx: succeed/fail/==/==-check + conj2/disj2 +
variadic mk-conj/mk-disj. ==-check is the opt-in occurs-checked variant.

Forced-rename note: SX has a host primitive `bind` that silently shadows
user-level defines, so all stream/goal operators are mk-prefixed. Recorded
in feedback memory.

82/82 tests cumulative (48 unify + 34 goals).

lib/minikanren/goals.sx

@@ -0,0 +1,58 @@
+;; lib/minikanren/goals.sx — Phase 2 piece B: core goals.
+;;
+;; A goal is a function (fn (s) → stream-of-substitutions).
+;; Goals built here:
+;;   succeed         — always returns (unit s)
+;;   fail            — always returns mzero
+;;   ==              — unifies two terms; succeeds with a singleton, else fails
+;;   ==-check        — opt-in occurs-checked equality
+;;   conj2 / mk-conj — sequential conjunction of goals
+;;   disj2 / mk-disj — interleaved disjunction of goals (raw — `conde` adds
+;;                     the implicit-conj-per-clause sugar in a later commit)
+
+(define succeed (fn (s) (unit s)))
+
+(define fail (fn (s) mzero))
+
+(define
+  ==
+  (fn
+    (u v)
+    (fn
+      (s)
+      (let ((s2 (mk-unify u v s))) (if (= s2 nil) mzero (unit s2))))))
+
+(define
+  ==-check
+  (fn
+    (u v)
+    (fn
+      (s)
+      (let ((s2 (mk-unify-check u v s))) (if (= s2 nil) mzero (unit s2))))))
+
+(define conj2 (fn (g1 g2) (fn (s) (mk-bind (g1 s) g2))))
+
+(define disj2 (fn (g1 g2) (fn (s) (mk-mplus (g1 s) (g2 s)))))
+
+;; Fold goals in a list. (mk-conj-list ()) ≡ succeed; (mk-disj-list ()) ≡ fail.
+(define
+  mk-conj-list
+  (fn
+    (gs)
+    (cond
+      ((empty? gs) succeed)
+      ((empty? (rest gs)) (first gs))
+      (:else (conj2 (first gs) (mk-conj-list (rest gs)))))))
+
+(define
+  mk-disj-list
+  (fn
+    (gs)
+    (cond
+      ((empty? gs) fail)
+      ((empty? (rest gs)) (first gs))
+      (:else (disj2 (first gs) (mk-disj-list (rest gs)))))))
+
+(define mk-conj (fn (&rest gs) (mk-conj-list gs)))
+
+(define mk-disj (fn (&rest gs) (mk-disj-list gs)))