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/test/bim/module/model/test_wall_header_refresh.py

@@ -75,7 +75,7 @@ def test_geom_generation_invalidates_wall_geom_cache():
     sentinel_a = {"length": 1.0, "height": 2.0, "x_angle": 0.0}
     sentinel_b = {"length": 1.5, "height": 2.5, "x_angle": 0.0}
 
-    with patch.object(wall_mod, "_read_wall_geometry", side_effect=[sentinel_a, sentinel_b]):
+    with patch.object(tool.Wall, "read_geometry", side_effect=[sentinel_a, sentinel_b]):
         first = wall_mod._get_wall_geom_cached(group, fake_obj)
         assert first is sentinel_a
         # Same call without a generation bump must hit the cache (no extra read).