relations: bulk lifecycle — relate-many! + unrelate-node! cascade cleanup + 12 tests

unrelate-node! retracts every local edge touching a node (all kinds, both
directions); leaves federated peer links alone. 147/147.

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

lib/relations/conformance.conf

@@ -30,4 +30,5 @@ SUITES=(
   "shape:lib/relations/tests/shape.sx:(relations-shape-tests-run!)"
   "tree:lib/relations/tests/tree.sx:(relations-tree-tests-run!)"
   "routes:lib/relations/tests/routes.sx:(relations-routes-tests-run!)"
+  "bulk:lib/relations/tests/bulk.sx:(relations-bulk-tests-run!)"
 )