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_entity_corner_ops.py

@@ -4,7 +4,6 @@ Backend coverage: ezdxf only. COM tests need a live AutoCAD instance and are
 skipped unless explicitly enabled.
 """
 
-import math
 import pytest
 
 pytestmark = pytest.mark.asyncio
@@ -151,7 +150,7 @@ class TestChamfer:
         # Tangent points: (96,0) and (100,4) in some order.
         pts = sorted([tuple(s[:2]), tuple(e[:2])])
         expected = sorted([(96.0, 0.0), (100.0, 4.0)])
-        for got, exp in zip(pts, expected):
+        for got, exp in zip(pts, expected, strict=True):
             assert _close(got[0], exp[0]) and _close(got[1], exp[1])
 
     async def test_chamfer_asymmetric(self, backend):
@@ -162,7 +161,7 @@ class TestChamfer:
         e = cl.properties["end"]
         pts = sorted([tuple(s[:2]), tuple(e[:2])])
         expected = sorted([(96.0, 0.0), (100.0, 6.0)])
-        for got, exp in zip(pts, expected):
+        for got, exp in zip(pts, expected, strict=True):
             assert _close(got[0], exp[0]) and _close(got[1], exp[1])
 
     async def test_chamfer_trims_source_lines(self, backend):