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/tests/test_fbx_export_preset.py

@@ -0,0 +1,70 @@
+"""Tests for Tools/fbx_export_preset.py.
+
+The preset module keeps its argument set in plain dicts and builds it in
+`preset_kwargs()`, so the export contract can be checked without starting
+Blender. Only `export_static`/`export_character` import `bpy`.
+"""
+
+import os
+import sys
+import unittest
+
+ROOT = os.path.abspath(os.path.join(os.path.dirname(__file__), "..", ".."))
+sys.path.insert(0, os.path.join(ROOT, "Tools"))
+
+import fbx_export_preset as preset  # noqa: E402
+
+
+class MetricPresetTest(unittest.TestCase):
+    def test_scale_and_axis_contract(self):
+        # These three are the project's recorded conversion contract: metres ->
+        # centimetres by a factor of exactly 100, east +X, north -Y, up +Z.
+        kwargs = preset.preset_kwargs()
+        self.assertTrue(kwargs["apply_unit_scale"])
+        self.assertEqual(kwargs["apply_scale_options"], "FBX_SCALE_NONE")
+        self.assertEqual(kwargs["axis_forward"], "-Y")
+        self.assertEqual(kwargs["axis_up"], "Z")
+
+    def test_smoothing_groups_are_exported(self):
+        # "OFF" (the pre-refactor value) drops smoothing groups and every Unreal
+        # import warns about it, with round parts faceted.
+        self.assertEqual(preset.preset_kwargs()["mesh_smooth_type"], "FACE")
+
+    def test_selection_flag(self):
+        self.assertFalse(preset.preset_kwargs()["use_selection"])
+        self.assertTrue(preset.preset_kwargs(selection=True)["use_selection"])
+
+    def test_object_types_are_meshes_only(self):
+        self.assertEqual(preset.preset_kwargs()["object_types"], {"MESH"})
+
+
+class CharacterPresetTest(unittest.TestCase):
+    def test_character_exports_armature_and_mesh(self):
+        kwargs = preset.preset_kwargs("ue_character_v1")
+        self.assertEqual(kwargs["object_types"], {"ARMATURE", "MESH"})
+        self.assertFalse(kwargs["add_leaf_bones"])
+        self.assertEqual(kwargs["path_mode"], "COPY")
+        self.assertFalse(kwargs["embed_textures"])
+
+    def test_character_keeps_the_scale_contract(self):
+        kwargs = preset.preset_kwargs("ue_character_v1")
+        self.assertEqual(kwargs["apply_scale_options"], "FBX_SCALE_NONE")
+        self.assertEqual(kwargs["mesh_smooth_type"], "FACE")
+
+
+class OverrideTest(unittest.TestCase):
+    def test_overrides_win(self):
+        kwargs = preset.preset_kwargs(mesh_smooth_type="OFF")
+        self.assertEqual(kwargs["mesh_smooth_type"], "OFF")
+
+    def test_unknown_preset_raises(self):
+        with self.assertRaises(KeyError):
+            preset.preset_kwargs("ue_imperial_v9")
+
+    def test_filepath_must_not_be_an_override(self):
+        with self.assertRaises(ValueError):
+            preset.preset_kwargs(filepath="/tmp/x.fbx")
+
+
+if __name__ == "__main__":
+    unittest.main()