Implement flexible entity relation system (Phases A–E)

Declarative relation registry via defrelation s-expressions with
cardinality enforcement (one-to-one, one-to-many, many-to-many),
registry-aware relate/unrelate/can-relate API endpoints, generic
container-nav fragment, and relation-driven UI components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

relations/bp/__init__.py

@@ -1,2 +1,3 @@
 from .data.routes import register as register_data
 from .actions.routes import register as register_actions
+from .fragments.routes import register as register_fragments