feat(styles): dimension, text and multileader styles on both engines; $DIMSTYLE/$TEXTSTYLE reach new entities headlessly

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

backends/contracts/__init__.py

@@ -24,6 +24,7 @@ from backends.contracts.linetypes import LinetypeContract
 from backends.contracts.premium import PremiumContract
 from backends.contracts.settings import SettingsContract
 from backends.contracts.solids import SolidContract
+from backends.contracts.styles import StylesContract
 from backends.contracts.system import SystemContract
 from backends.contracts.transactions import TransactionContract
 from backends.contracts.view import ViewContract
@@ -45,6 +46,7 @@ __all__ = [
     "PremiumContract",
     "SettingsContract",
     "SolidContract",
+    "StylesContract",
     "SystemContract",
     "TransactionContract",
     "ViewContract",