relations: Phase 3 path explanation + distance + mixed-kind reachability (explain.sx, reach_any) + 24 tests

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

lib/relations/conformance.conf

@@ -17,9 +17,11 @@ PRELOADS=(
   lib/relations/schema.sx
   lib/relations/engine.sx
   lib/relations/api.sx
+  lib/relations/explain.sx
 )
 
 SUITES=(
   "direct:lib/relations/tests/direct.sx:(relations-direct-tests-run!)"
   "reach:lib/relations/tests/reach.sx:(relations-reach-tests-run!)"
+  "path:lib/relations/tests/path.sx:(relations-path-tests-run!)"
 )