feat(environment): multi-document on both engines — ezdxf document registry, document_list/activate/close, drawing_close folded in

EnvironmentContract carries every member of the track E interface; the two
drawing_properties_* members are @capability('dwgprops') in this worktree only
and become @abstractmethod when group C's implementations merge.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FKFStjEFzpGyLq5bMpWxTd

backends/contracts/__init__.py

@@ -16,6 +16,7 @@ from backends.contracts.drawing import DrawingContract
 from backends.contracts.entity_creation import EntityCreationContract
 from backends.contracts.entity_modification import EntityModificationContract
 from backends.contracts.entity_query import EntityQueryContract
+from backends.contracts.environment import EnvironmentContract
 from backends.contracts.gdt import GdtContract
 from backends.contracts.identity import IdentityContract
 from backends.contracts.layers import LayerContract
@@ -37,6 +38,7 @@ __all__ = [
     "EntityCreationContract",
     "EntityModificationContract",
     "EntityQueryContract",
+    "EnvironmentContract",
     "GdtContract",
     "IdentityContract",
     "LayerContract",