Discard previews on IFC save + harden preview-active gate

Save-path:
* SaveProject._execute (project/operator.py) now calls
  preview_base.discard_pending_previews(context.scene) right after
  tool.Parametric.commit_pending_edits(). Previews are session-
  transient — discard rather than commit. Sibling gizmo polls gate
  on each preview's is_active flag; a stuck flag persisted through
  the save would silently hide them on reload. Mirrors the pattern
  already in gizmos-8088.

Preview-active gate hardening:
* preview_base.get_preview_props tolerates contexts without a
  ``scene`` attribute. Pre-existing tests use SimpleNamespace mocks
  for the context; the previous getattr(context.scene, ...) raised
  AttributeError before the inner default kicked in.

Test update:
* test_wall_header_refresh.test_geom_generation_invalidates_wall_geom_cache
  patches tool.Wall.read_geometry instead of the now-deleted local
  wall._read_wall_geometry (commit 7e5e7b8d6 migrated the call site).

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/project/operator.py

@@ -63,6 +63,7 @@ import bonsai.core.project as core
 import bonsai.tool as tool
 from bonsai.bim import export_ifc, import_ifc
 from bonsai.bim.ifc import IfcStore
+from bonsai.bim.module.model import preview_base
 from bonsai.bim.module.model.decorator import FaceAreaDecorator, PolylineDecorator
 from bonsai.bim.module.model.polyline import PolylineOperator
 from bonsai.bim.module.project.data import LinksData, ProjectLibraryData
@@ -1936,6 +1937,10 @@ class ExportIFC(bpy.types.Operator, ExportHelper):
 
     def _execute(self, context):
         committed, failed_commits = tool.Parametric.commit_pending_edits()
+        # Previews are session-transient — discard rather than commit. Sibling
+        # gizmo polls gate on each preview's is_active flag, and a stuck flag
+        # persisted through the save would silently hide them on reload.
+        preview_base.discard_pending_previews(context.scene)
         # Suffix is appended to the IFC save-success report below so the auto-commit
         # info isn't immediately overwritten by the success message in Blender's
         # status bar (only the latest self.report({"INFO"}, ...) sticks).