fix(correctness-bundle): I16 correctness fixes + dead code removal + ruff clean

N1: COM hatch non-associative (AddHatch associativity=False)
N2: ezdxf refuses DWG saves with clear error; server.py derives fmt from extension
R19: dimension_aligned dist→distance; dimension_angular correct add_angular_dim_2l args (base/line1/line2/location)
R9: polar array 360° no-duplicate (divisor=n when fill_angle%360<ε)
R13: entity_offset side_x/side_y honored in ezdxf via cross-product (LINE) and distance-to-center (CIRCLE)
R18: drawing_new bootstrap failure surfaces status="degraded" instead of swallowing silently
Dead code: engineering/section.py, gear.generate_tooth_profile, CommandResult, _ACI_NAMES, set_layer_active, ensure_linetypes, ComBackend._batch_mode instance attr, engineering/__init__ build-seam comment
Ruff: 38 auto-fixed + 5 manual → codebase is now ruff-clean; CI lint is now blocking

184 tests pass.

backends/__init__.py

@@ -1,11 +1,10 @@
 """AutoCAD MCP Pro – backend package."""
-from .base import AutoCADBackend, BlockInfo, CommandResult, DrawingInfo, EntityInfo, LayerInfo
+from .base import AutoCADBackend, BlockInfo, DrawingInfo, EntityInfo, LayerInfo
 from .com_backend import ComBackend
 from .ezdxf_backend import EzdxfBackend
 
 __all__ = [
     "AutoCADBackend",
-    "CommandResult",
     "EntityInfo",
     "LayerInfo",
     "BlockInfo",