relations: route enumeration — all-paths (all simple directed paths a->b) + 9 tests

Cycle-safe DFS in explain.sx, complements shortest-path relations-path. 135/135.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/relations/explain.sx

@@ -69,6 +69,28 @@
 
 ;; --- current-db convenience layer ---
 
+(define
+  relations-ap-dfs
+  (fn
+    (db b kind path node)
+    (if
+      (= node b)
+      (list path)
+      (relations-concat-map
+        (fn
+          (nbr)
+          (if
+            (relations-eng-member? nbr path)
+            (list)
+            (relations-ap-dfs db b kind (append path (list nbr)) nbr)))
+        (relations-children-of db node kind)))))
+
+(define
+  relations-all-paths
+  (fn
+    (db a b kind)
+    (if (= a b) (list (list a)) (relations-ap-dfs db b kind (list a) a))))
+
 (define
   relations/path
   (fn (a b kind) (relations-path (relations-ensure-db!) a b kind)))
@@ -84,3 +106,7 @@
 (define
   relations/reachable-any?
   (fn (a b) (relations-reachable-any? (relations-ensure-db!) a b)))
+
+(define
+  relations/all-paths
+  (fn (a b kind) (relations-all-paths (relations-ensure-db!) a b kind)))