Fix wall edit lifecycle + drain wall_offset_gizmos cache on load

Bundled bug fixes + the forward-compat AST guard that prevents the
underlying class of bug from coming back.

* bim/module/model/wall.py: FinishEditingWall._execute early-returns
  CANCELLED when props.is_editing is False. Without this guard, a
  failed enable (e.g. on a wall without IfcMaterialLayerSetUsage)
  leaves is_editing False but a press on finish still walked the
  sub-ops below, which dereferenced layer-set-dependent state and
  crashed.

* tool/model.py: Model.offset_wall now guards against
  ifcopenshell.util.element.get_material returning None before
  calling .is_a("IfcMaterialLayerSetUsage"). Fixes the pre-existing
  test/bim/module/model/test_wall_header_refresh.py crash that has
  been the only failing test in the wall lane since this branch
  started.

* bim/handler.py: _apply_save_file_invariants drains
  wall_offset_gizmos.clear_caches() on load_post. The module-scope
  GenerationKeyedCache instance survives the .blend reload; without
  the drain the cache may serve entries whose bpy_struct references
  point into the freed bpy.data of the previous file.

* test/bim/test_handler_forward_compat.py: AST-walk test that
  enumerates every bim/module/model/*.py source declaring both a
  module-scope GenerationKeyedCache assignment AND a top-level
  clear_caches function, and asserts each module appears as a
  <module>.clear_caches() call in _apply_save_file_invariants. Pins
  the contract: any future module-scope geom cache that exposes
  clear_caches must wire into the load_post drain.

* test/bim/feature/model.feature + test/bim/test_feature.py: wall
  edit-lifecycle scenarios switch from "add cube + assign as
  IfcWallType" to "load the demo construction library + add an
  occurrence of the WAL100 wall type", so the parametric edit runs
  against a real LAYER2 wall with IfcMaterialLayerSetUsage rather
  than a vanilla-mesh promotion that lacks one. The demo-library
  step also picks the schema-matching library file (IFC2X3 /
  IFC4 / IFC4X3) so the appended types remain valid across schemas.
  Door saved-height assertion updates from 2.5 → 2500 to reflect
  that BBIM_Door pset stores project units (METRIC_MM in the
  empty-project fixture).

Generated with the assistance of an AI coding tool.

src/bonsai/test/bim/test_feature.py

@@ -1764,7 +1764,17 @@ def i_load_the_ifc_test_file(filepath):
 @given("I load the demo construction library")
 @when("I load the demo construction library")
 def i_add_a_construction_library():
-    lib_path = "./bonsai/bim/data/libraries/IFC4 Demo Library.ifc"
+    # Pick the library file whose schema matches the current project so the
+    # appended types are valid (IFC2X3-vs-IFC4 entity attributes differ).
+    schema_to_library = {
+        "IFC2X3": "IFC2X3 Demo Library.ifc",
+        "IFC4": "IFC4 Demo Library.ifc",
+        "IFC4X3": "IFC4X3 Demo Library.ifc",
+        "IFC4X3_ADD2": "IFC4X3 Demo Library.ifc",
+    }
+    schema = tool.Ifc.get().schema
+    lib_name = schema_to_library.get(schema, "IFC4 Demo Library.ifc")
+    lib_path = f"./bonsai/bim/data/libraries/{lib_name}"
     bpy.ops.bim.select_library_file(filepath=lib_path, append_all=True)