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/fbx_export_preset.py

@@ -0,0 +1,94 @@
+#!/usr/bin/env python3
+"""The one FBX export preset for this project ("ue_metric_v1"/"ue_character_v1").
+
+Before this module the same `bpy.ops.export_scene.fbx(...)` argument block was
+copied into six scripts (hero, frontage, north buildings, site, scale test, MPFB
+character). They had already drifted only by accident, and every one of them
+carried `mesh_smooth_type="OFF"` — which is why Unreal logged "No smoothing group
+information was found for this mesh" on every import and round parts rendered
+faceted. One preset, one place to fix that.
+
+Use from inside Blender:
+
+    import sys, os
+    sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "Tools"))
+    import fbx_export_preset as preset
+
+    preset.export_static(path)                      # whole scene, MESH only
+    preset.export_static(path, selection=True)      # selected objects only
+    preset.export_character(path)                   # armature + mesh, character flags
+
+`preset_kwargs()` is pure (no `bpy`), so the argument set can be unit-tested
+without Blender — see Tools/tests/test_fbx_export_preset.py.
+
+Why these values:
+* `FBX_SCALE_NONE` + `apply_unit_scale` keeps Blender metres → Unreal centimetres
+  at exactly ×100, verified by the three-cube scale test (1 m → 100.00 uu).
+* `axis_forward="-Y"`, `axis_up="Z"` is the project's recorded mapping: east → +X,
+  north → −Y, up → +Z.
+* `mesh_smooth_type="FACE"` exports smoothing groups. `"OFF"` (the old value)
+  drops them, so Unreal rebuilds flat normals and every cylinder is faceted.
+* `use_mesh_modifiers=True` bakes the bevel/modifier stack at export; the source
+  `.blend` stays non-destructive.
+"""
+
+from __future__ import annotations
+
+# Shared by every mesh export. Values are the recorded ue_metric_v1 preset.
+_METRIC = {
+    "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": "FACE",
+    "path_mode": "AUTO",
+    "bake_space_transform": False,
+}
+
+# Character exports differ only in the object set and the armature/path flags.
+_CHARACTER = dict(
+    _METRIC,
+    object_types={"ARMATURE", "MESH"},
+    add_leaf_bones=False,
+    path_mode="COPY",
+    embed_textures=False,
+)
+
+PRESETS = {"ue_metric_v1": _METRIC, "ue_character_v1": _CHARACTER}
+
+
+def preset_kwargs(preset: str = "ue_metric_v1", selection: bool = False, **overrides) -> dict:
+    """Return the `bpy.ops.export_scene.fbx` argument dict (no `bpy` needed).
+
+    `preset` selects the base set, `selection` decides whether only selected
+    objects are exported, and `overrides` wins last for the rare special case.
+    """
+    if preset not in PRESETS:
+        raise KeyError(f"unknown preset {preset!r}; known: {sorted(PRESETS)}")
+    kwargs = dict(PRESETS[preset])
+    kwargs["use_selection"] = bool(selection)
+    kwargs.update(overrides)
+    if "filepath" in kwargs:
+        raise ValueError("pass the path to export_static/export_character, not as an override")
+    return kwargs
+
+
+def _export(path: str, preset: str, selection: bool, overrides: dict) -> str:
+    import bpy  # imported here so `preset_kwargs` stays usable without Blender
+
+    kwargs = preset_kwargs(preset, selection=selection, **overrides)
+    bpy.ops.export_scene.fbx(filepath=path, **kwargs)
+    return path
+
+
+def export_static(path: str, selection: bool = False, **overrides) -> str:
+    """Export meshes with the ue_metric_v1 preset. Returns the path."""
+    return _export(path, "ue_metric_v1", selection, overrides)
+
+
+def export_character(path: str, selection: bool = False, **overrides) -> str:
+    """Export armature + mesh with the ue_character_v1 preset. Returns the path."""
+    return _export(path, "ue_character_v1", selection, overrides)