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/hero_facade_build.py

@@ -33,6 +33,9 @@ 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
+
 
 # --- measured / derived ------------------------------------------------------
 FRONTAGE_W = 25.75
@@ -306,24 +309,10 @@ def build():
 
 
 def export_fbx(path):
+    # Preset lives in Tools/fbx_export_preset.py (ue_metric_v1).
     os.makedirs(os.path.dirname(path), exist_ok=True)
     # Named export preset "ue_metric_v1" - same settings as Tools/scale_test_export.py.
-    bpy.ops.export_scene.fbx(
-        filepath=path,
-        use_selection=False,
-        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,
-        # "FACE" exports smoothing groups; without it every import logs
-        # "No smoothing group information was found for this mesh".
-        mesh_smooth_type="FACE",
-        path_mode="AUTO",
-        bake_space_transform=False,
-    )
+    fbx_preset.export_static(path)
     print(f"[hero] exported FBX preset ue_metric_v1 -> {path}")