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/base.py

@@ -25,6 +25,7 @@ from backends.contracts import (
     EntityCreationContract,
     EntityModificationContract,
     EntityQueryContract,
+    EnvironmentContract,
     GdtContract,
     IdentityContract,
     LayerContract,
@@ -184,6 +185,7 @@ class AutoCADBackend(
     PremiumContract,
     GdtContract,
     SettingsContract,
+    EnvironmentContract,
 ):
     """All backends implement this interface.