relations: shape queries (siblings, in/out-degree, undirected connected?) computed in SX + 18 tests

Keep the Datalog ruleset minimal — every dl-query re-saturates, so shape
queries are SX BFS over erel, not extra closures. 110/110.

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

lib/relations/api.sx

@@ -84,3 +84,19 @@
 (define
   relations/acyclic?
   (fn (kind) (relations-acyclic? (relations-ensure-db!) kind)))
+
+(define
+  relations/siblings
+  (fn (node kind) (relations-siblings (relations-ensure-db!) node kind)))
+
+(define
+  relations/out-degree
+  (fn (node kind) (relations-out-degree (relations-ensure-db!) node kind)))
+
+(define
+  relations/in-degree
+  (fn (node kind) (relations-in-degree (relations-ensure-db!) node kind)))
+
+(define
+  relations/connected?
+  (fn (a b kind) (relations-connected? (relations-ensure-db!) a b kind)))