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/gear.py

@@ -71,58 +71,6 @@ def _arc_points(
     return pts
 
 
-def generate_tooth_profile(
-    module: float,
-    teeth: int,
-    pressure_angle: float = 20.0,
-    n_points_per_flank: int = 40,
-) -> list[tuple[float, float]]:
-    """Single tooth profile: right flank, tip arc, mirrored left flank."""
-    pa = math.radians(pressure_angle)
-    pitch_r = module * teeth / 2.0
-    base_r = pitch_r * math.cos(pa)
-    outer_r = pitch_r + module
-    root_r = pitch_r - 1.25 * module
-
-    # Tooth half-thickness angle on the pitch circle.
-    s = math.pi * module / 2.0
-    theta_pitch = s / pitch_r / 2.0
-
-    # Find involute parameter at pitch circle so we can shift the flank to be
-    # symmetric about the tooth centerline (y=0 axis after rotation).
-    if outer_r > base_r:
-        t_pitch = math.sqrt((pitch_r / base_r) ** 2 - 1.0) if pitch_r > base_r else 0.0
-    else:
-        t_pitch = 0.0
-    inv_pitch = math.tan(pa) - pa  # involute function at pressure angle
-    # The flank generated by `generate_involute_flank` starts at theta=0 on the
-    # base circle. The involute point at the pitch radius makes angle inv_pitch
-    # (approx). We rotate the flank by -inv_pitch + theta_pitch so the pitch
-    # crossing sits at +theta_pitch above the tooth-center axis.
-    rot = -inv_pitch + theta_pitch
-
-    right_flank = generate_involute_flank(base_r, root_r, outer_r, n_points_per_flank)
-    right_flank = _rotate_points(right_flank, rot)
-
-    # Left flank by mirroring across the X axis (tooth centerline).
-    left_flank = [(x, -y) for x, y in right_flank]
-
-    # Tip arc connects the outermost right-flank point to the outermost left-flank point.
-    tip_right = right_flank[-1]
-    tip_left = left_flank[-1]
-    a_right = math.atan2(tip_right[1], tip_right[0])
-    a_left = math.atan2(tip_left[1], tip_left[0])
-    if a_left > a_right:
-        a_left -= 2 * math.pi
-    tip_arc = _arc_points(0.0, 0.0, outer_r, a_right, a_left, n=5)
-
-    profile: list[tuple[float, float]] = []
-    profile.extend(right_flank)
-    profile.extend(tip_arc[1:-1])
-    profile.extend(reversed(left_flank))
-    return profile
-
-
 def generate_full_gear_outline(
     module: float,
     teeth: int,
@@ -200,7 +148,7 @@ def generate_full_gear_outline(
 
 
 async def _draw_helix_symbol(
-    backend: "AutoCADBackend",
+    backend: AutoCADBackend,
     *,
     cx: float,
     cy: float,
@@ -237,7 +185,7 @@ async def _draw_helix_symbol(
 
 
 async def draw_helical_gear_front_view(
-    backend: "AutoCADBackend",
+    backend: AutoCADBackend,
     *,
     module: float,
     teeth: int,
@@ -330,7 +278,7 @@ async def draw_helical_gear_front_view(
 
 
 async def draw_spur_gear_front_view(
-    backend: "AutoCADBackend",
+    backend: AutoCADBackend,
     *,
     module: float,
     teeth: int,
@@ -356,7 +304,7 @@ async def draw_spur_gear_front_view(
 
 
 async def draw_gear_section_aa(
-    backend: "AutoCADBackend",
+    backend: AutoCADBackend,
     *,
     gear_metadata: dict,
     x_offset: float,