refactor(tools): one FBX export preset instead of six copies

The bpy.ops.export_scene.fbx argument block was duplicated in six scripts (hero,
block frontage, north buildings, site, scale test, MPFB character) and all six
carried mesh_smooth_type="OFF", which drops smoothing groups and made every
Unreal import warn and every round part faceted.

Tools/fbx_export_preset.py now owns the presets:
- preset_kwargs(preset, selection, **overrides) is bpy-free and unit-tested;
- export_static()/export_character() are thin wrappers that import bpy lazily;
- ue_metric_v1 and ue_character_v1 keep the recorded scale contract
  (FBX_SCALE_NONE, apply_unit_scale, -Y forward / Z up) and use FACE smoothing.

Callers switched: hero_facade_build, export_block_frontage, export_north_buildings,
scale_test_export, mpfb_character_export, Art/blender/site/export_site_fbx.

Verified by re-exporting every asset through the preset and re-importing all of
them: 0 smoothing warnings on site, frontage and north buildings; packaging and
-qaselftest all PASS (walk, transect 0 holes, wall probe 0/35, facade probe 0/9).
49 tool tests pass (9 new for the preset contract).

Tools/export_north_buildings.py

@@ -13,6 +13,9 @@ import os
 import sys
 
 import bpy
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+import fbx_export_preset as fbx_preset  # noqa: E402
+
 
 BUILDINGS = ("TCL Chinese Theatre", "Madame Tussaud's")
 
@@ -50,20 +53,7 @@ def main():
         bpy.context.view_layer.objects.active = new_obj
 
         path = os.path.join(args.out, name.replace(" ", "_").replace("'", "") + ".fbx")
-        bpy.ops.export_scene.fbx(
-            filepath=path,
-            use_selection=True,
-            object_types={"MESH"},
-            apply_unit_scale=True,
-            apply_scale_options="FBX_SCALE_NONE",
-            axis_forward="-Y",
-            axis_up="Z",
-            use_space_transform=True,
-            use_mesh_modifiers=True,
-            mesh_smooth_type="OFF",
-            path_mode="AUTO",
-            bake_space_transform=False,
-        )
+        fbx_preset.export_static(path, selection=True)
         world = new_obj.matrix_world
         zs = [(world @ v.co).z for v in new_mesh.vertices]
         print(f"[north] {name:22} verts {len(new_mesh.vertices):5}  z {min(zs):+.2f}..{max(zs):+.2f} m "