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/scoreboard.json

@@ -1,13 +1,14 @@
 {
   "lang": "relations",
-  "total_passed": 92,
+  "total_passed": 110,
   "total_failed": 0,
-  "total": 92,
+  "total": 110,
   "suites": [
     {"name":"direct","passed":22,"failed":0,"total":22},
     {"name":"reach","passed":24,"failed":0,"total":24},
     {"name":"path","passed":24,"failed":0,"total":24},
-    {"name":"fed","passed":22,"failed":0,"total":22}
+    {"name":"fed","passed":22,"failed":0,"total":22},
+    {"name":"shape","passed":18,"failed":0,"total":18}
   ],
-  "generated": "2026-06-07T12:14:10+00:00"
+  "generated": "2026-06-07T12:55:58+00:00"
 }