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/layers.py

@@ -71,7 +71,7 @@ LAYER_SET_REGISTRY: dict[str, list[tuple[str, int, str, float, str]]] = {
 STANDARD_LINETYPES: list[str] = ["CENTER", "HIDDEN", "PHANTOM", "DASHED", "DASHDOT"]
 
 
-async def ensure_standard_linetypes(backend: "AutoCADBackend") -> dict[str, str]:
+async def ensure_standard_linetypes(backend: AutoCADBackend) -> dict[str, str]:
     """Idempotently load STANDARD_LINETYPES via backend.linetype_load."""
     try:
         existing = {ln.lower() for ln in await backend.linetype_list()}
@@ -92,13 +92,13 @@ async def ensure_standard_linetypes(backend: "AutoCADBackend") -> dict[str, str]
     return results
 
 
-async def ensure_engineering_layers(backend: "AutoCADBackend") -> dict[str, str]:
+async def ensure_engineering_layers(backend: AutoCADBackend) -> dict[str, str]:
     """Idempotently create every layer in ENGINEERING_LAYERS via backend.layer_create."""
     return await apply_layer_set(backend, "mech")
 
 
 async def apply_layer_set(
-    backend: "AutoCADBackend", standard: str = "mech",
+    backend: AutoCADBackend, standard: str = "mech",
 ) -> dict[str, str]:
     """Idempotently apply a named layer set ('mech', 'pid', 'iso13567')."""
     layer_set = LAYER_SET_REGISTRY.get(standard)