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/conformance.conf

@@ -26,4 +26,5 @@ SUITES=(
   "reach:lib/relations/tests/reach.sx:(relations-reach-tests-run!)"
   "path:lib/relations/tests/path.sx:(relations-path-tests-run!)"
   "fed:lib/relations/tests/fed.sx:(relations-fed-tests-run!)"
+  "shape:lib/relations/tests/shape.sx:(relations-shape-tests-run!)"
 )