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.

tests/test_dimensions.py

@@ -13,14 +13,12 @@ async def test_dimension_linear(backend):
     assert info.type == "DIMENSION"
 
 
-@pytest.mark.xfail(reason="ezdxf API parameter mismatch in add_aligned_dim", strict=False)
 async def test_dimension_aligned(backend):
     info = await backend.dimension_aligned(0, 0, 100, 50, 50, 60)
     assert info.handle
     assert info.type == "DIMENSION"
 
 
-@pytest.mark.xfail(reason="ezdxf API parameter mismatch in add_angular_dim_2l", strict=False)
 async def test_dimension_angular(backend):
     info = await backend.dimension_angular(0, 0, 100, 0, 0, 100, 50, 50)
     assert info.handle