mk: phase 5A — conda, soft-cut without onceo

conda-try mirrors condu-try but on the chosen clause it (mk-bind
(head-goal s) (rest-conj)) — all head answers flow through. condu by
contrast applies rest-conj to (first peek), keeping only one head
answer.

7 new tests covering: first-non-failing-wins, skip-failing-head, all-fail,
no-clauses, the conda-vs-condu divergence (`(1 2)` vs `(1)`), rest-goals
running on every head answer, and the soft-cut no-fallthrough property.

169/169 cumulative.

lib/minikanren/conda.sx

@@ -0,0 +1,42 @@
+;; lib/minikanren/conda.sx — Phase 5 piece A: `conda`, the soft-cut.
+;;
+;;   (conda (g0 g ...) (h0 h ...) ...)
+;;     — first clause whose head g0 produces ANY answer wins; ALL of g0's
+;;       answers are then conj'd with the rest of that clause; later
+;;       clauses are NOT tried.
+;;     — differs from condu only in not wrapping g0 in onceo: condu
+;;       commits to the SINGLE first answer, conda lets the head's full
+;;       answer-set flow into the rest of the clause.
+;;       (Reasoned Schemer chapter 10; Byrd 5.3.)
+
+(define
+  conda-try
+  (fn
+    (clauses s)
+    (cond
+      ((empty? clauses) mzero)
+      (:else
+        (let
+          ((cl (first clauses)))
+          (let
+            ((head-goal (first cl)) (rest-goals (rest cl)))
+            (let
+              ((peek (stream-take 1 (head-goal s))))
+              (if
+                (empty? peek)
+                (conda-try (rest clauses) s)
+                (mk-bind (head-goal s) (mk-conj-list rest-goals))))))))))
+
+(defmacro
+  conda
+  (&rest clauses)
+  (quasiquote
+    (fn
+      (s)
+      (conda-try
+        (list
+          (splice-unquote
+            (map
+              (fn (cl) (quasiquote (list (splice-unquote cl))))
+              clauses)))
+        s))))