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.

engineering/__init__.py

@@ -6,7 +6,6 @@ from .gear import (
     draw_spur_gear_front_view,
     generate_full_gear_outline,
     generate_involute_flank,
-    generate_tooth_profile,
     involute_xy,
 )
 from .keyway import (
@@ -32,23 +31,8 @@ from .plan_spec import (
     SheetSize,
     SnapType,
 )
-from .section import apply_section_hatch, cutting_plane_line
-
-# Agent B contributes titleblock + validator. Imports are guarded so this
-# package remains usable while B is still in flight; once those modules land
-# the symbols flow through transparently.
-try:
-    from .titleblock import TitleBlockMetadata, apply_iso_a3_titleblock
-except ImportError:
-    TitleBlockMetadata = None  # type: ignore[assignment]
-    apply_iso_a3_titleblock = None  # type: ignore[assignment]
-
-try:
-    from .validator import DrawingValidator, ValidationFinding, ValidationResult
-except ImportError:
-    DrawingValidator = None  # type: ignore[assignment]
-    ValidationFinding = None  # type: ignore[assignment]
-    ValidationResult = None  # type: ignore[assignment]
+from .titleblock import TitleBlockMetadata, apply_iso_a3_titleblock
+from .validator import DrawingValidator, ValidationFinding, ValidationResult
 
 __all__ = [
     "draw_helical_gear_front_view",
@@ -56,14 +40,11 @@ __all__ = [
     "draw_gear_section_aa",
     "generate_full_gear_outline",
     "generate_involute_flank",
-    "generate_tooth_profile",
     "involute_xy",
     "draw_keyed_bore",
     "draw_keyway_section",
     "keyway_dimensions",
     "DIN6885_TABLE",
-    "cutting_plane_line",
-    "apply_section_hatch",
     "ENGINEERING_LAYERS",
     "STANDARD_LINETYPES",
     "ensure_engineering_layers",