relations: Phase 3 path explanation + distance + mixed-kind reachability (explain.sx, reach_any) + 24 tests

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

lib/relations/engine.sx

@@ -8,6 +8,9 @@
 ;;   reach(K,X,Y) :- rel(X,Y,K).                  ; one hop
 ;;   reach(K,X,Y) :- rel(X,Z,K), reach(K,Z,Y).    ; transitive
 ;;
+;; `reach_any` is the kind-agnostic closure (any edge, any kind) used for
+;; mixed-kind reachability — distinct from single-kind `reach`.
+;;
 ;; rnode collects the nodes touched by a kind; root/leaf are those with no
 ;; incoming / no outgoing edge (stratified negation over has_parent/has_child).
 ;; Cycles are ordinary data: `reach(K,X,X)` simply holds for nodes on a cycle —
@@ -18,6 +21,8 @@
   (quote
     ((reach K X Y <- (rel X Y K))
       (reach K X Y <- (rel X Z K) (reach K Z Y))
+      (reach_any X Y <- (rel X Y K))
+      (reach_any X Y <- (rel X Z K) (reach_any Z Y))
       (rnode K X <- (rel X Y K))
       (rnode K Y <- (rel X Y K))
       (has_parent K Y <- (rel X Y K))
@@ -55,6 +60,22 @@
     (db a b kind)
     (> (len (dl-query db (list (quote reach) kind a b))) 0)))
 
+;; Mixed-kind: descendants reachable from node over edges of ANY kind.
+(define
+  relations-descendants-any
+  (fn
+    (db node)
+    (relations-pluck
+      (dl-query db (list (quote reach_any) node (quote Y)))
+      :Y)))
+
+;; Mixed-kind: is b reachable from a over edges of ANY kind?
+(define
+  relations-reachable-any?
+  (fn
+    (db a b)
+    (> (len (dl-query db (list (quote reach_any) a b))) 0)))
+
 ;; Roots: nodes touched by kind with no incoming edge.
 (define
   relations-roots