relations: tree/DAG queries (common-ancestors, lca, topo-order) in SX + 16 tests

lib/relations/tree.sx over reach/ancestors/rnode — no new Datalog closures. 126/126.

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

lib/relations/conformance.conf

@@ -19,6 +19,7 @@ PRELOADS=(
   lib/relations/api.sx
   lib/relations/explain.sx
   lib/relations/federation.sx
+  lib/relations/tree.sx
 )
 
 SUITES=(
@@ -27,4 +28,5 @@ SUITES=(
   "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!)"
+  "tree:lib/relations/tests/tree.sx:(relations-tree-tests-run!)"
 )