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_meta_tools.py

@@ -187,9 +187,9 @@ class TestDimensionAuto:
         assert len(dims) == 2
 
     async def test_unknown_style_raises(self, backend):
-        l = await backend.entity_create_line(0, 0, 50, 0)
+        ln = await backend.entity_create_line(0, 0, 50, 0)
         with pytest.raises(RuntimeError, match="unknown style"):
-            await backend.dimension_auto([l.handle], style="diagonal")
+            await backend.dimension_auto([ln.handle], style="diagonal")
 
     async def test_empty_handles_returns_empty(self, backend):
         result = await backend.dimension_auto([], style="chain")