Decompose bim/handler.py load_post + install cache + discard hooks

Three concerns folded into ``load_post`` argue for separation:

1. Save-file invariants every load must re-establish (msgbus
   subscription, owner-settings, thumbnail cache, draft-flag healing,
   blend-warning flag, H5 lock probe).
2. User-preference-driven UI setup (toolbar, workspace, viewport
   shading, panel hijack, snap defaults).
3. Viewport overlay sync (every decorator's install/uninstall).

Pull each into its own function (``_apply_save_file_invariants`` /
``_apply_user_preferences`` / ``_install_viewport_overlays``). The
``load_post`` callback becomes a 3-line orchestrator. Each phase
is independently call-able from tests and from PR4 features that
need to re-trigger one phase without the others.

Two new hooks land with the decompose:

* ``tool.Parametric.heal_stale_edit_flags()`` + ``discard_pending_previews(scene)``
  fire in ``_apply_save_file_invariants``. The first clears
  object-level ``BIM<Name>Properties.is_editing`` flags that lost
  their backing IFC element across a load; the second clears
  scene-level ``BIMPreviewProperties.<x>.is_active`` so saved
  preview state never resurfaces with no UI to interact with it.

* ``install_decorator_cache_handlers`` / ``uninstall_decorator_cache_handlers``
  wrap the decorator install/install pass in
  ``_install_viewport_overlays``. The bump handlers append to
  ``depsgraph_update_post`` + ``undo_post`` + ``redo_post`` +
  ``load_post`` so the previous commit's ``TokenCache`` in
  ``tool.System.get_decoration_data`` finally invalidates on
  structural scene changes.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/handler.py

@@ -34,7 +34,11 @@ from mathutils import Vector
 import bonsai.bim
 import bonsai.core.model as core_model
 import bonsai.tool as tool
-from bonsai.bim.ifc import IfcStore
+from bonsai.bim.decorator_cache import (
+    install_decorator_cache_handlers,
+    uninstall_decorator_cache_handlers,
+)
+from bonsai.bim.ifc import IfcStore, get_cache_or_detect_lock
 from bonsai.bim.module.aggregate.decorator import AggregateDecorator
 from bonsai.bim.module.georeference.decorator import GeoreferenceDecorator
 from bonsai.bim.module.model.data import AuthoringData
@@ -43,6 +47,7 @@ from bonsai.bim.module.model.decorator import (
     SlabDirectionDecorator,
     WallAxisDecorator,
 )
+from bonsai.bim.module.model.preview_base import discard_pending_previews
 from bonsai.bim.module.nest.decorator import NestDecorator
 
 cwd = os.path.dirname(os.path.realpath(__file__))
@@ -378,8 +383,10 @@ def subscribe_to_viewport_shading_changes():
             )
 
 
-@persistent
-def load_post(scene):
+def _apply_save_file_invariants(scene: bpy.types.Scene) -> None:
+    """Invariants enforced on every load_post: msgbus subscription, IFC owner
+    settings, scene-bound caches, draft-flag healing, multi-instance lock probe,
+    and previews discarded so saved preview state never resurfaces on reopen."""
     global global_subscription_owner
     active_object_key = bpy.types.LayerObjects, "active"
     bpy.msgbus.subscribe_rna(
@@ -390,6 +397,24 @@ def load_post(scene):
     ifcopenshell.api.owner.settings.get_application = get_application
     AuthoringData.type_thumbnails = {}
 
+    tool.Parametric.heal_stale_edit_flags()
+    discard_pending_previews(scene)
+
+    if tool.Ifc.get() and bpy.data.is_saved:
+        props = tool.Blender.get_bim_props()
+        props.has_blend_warning = True
+
+    # Probe the H5 cooked-geometry cache so the multi-instance warning surfaces
+    # right after .blend load. Without this, the lock is only detected when a
+    # mutation triggers ``clear_cache`` — by which time the user has already
+    # made changes that may now conflict with the other Blender instance.
+    if tool.Ifc.get():
+        get_cache_or_detect_lock()
+
+
+def _apply_user_preferences() -> None:
+    """User-preference-driven UI setup: toolbar, BIM workspace, viewport shading
+    subscription, scene-panel hijack, tab layout, snap defaults."""
     preferences = tool.Blender.get_addon_preferences()
     if not preferences.should_setup_toolbar:
         tool.Blender.unregister_toolbar()
@@ -413,11 +438,21 @@ def load_post(scene):
         tool.Blender.override_scene_panel(panel)
     tool.Blender.setup_tabs()
 
-    if tool.Ifc.get() and bpy.data.is_saved:
-        props = tool.Blender.get_bim_props()
-        props.has_blend_warning = True
+    if preferences.should_use_snap and (scene := bpy.context.scene):
+        # Snapping is off by default in Blender, but in BIM, it's more useful to be on
+        scene.tool_settings.use_snap = True
+        # Match default Bonsai snaps
+        scene.tool_settings.snap_elements_base = {"EDGE", "EDGE_PERPENDICULAR", "VERTEX", "EDGE_MIDPOINT", "FACE"}
 
-    # Bonsai overlays
+    tool.Blender.sync_old_preferences()
+
+
+def _install_viewport_overlays() -> None:
+    """Sync every Bonsai viewport decorator to its enabled state.
+
+    Wrapped in uninstall/install of the decorator-cache bump handlers so a
+    decorator's own install path doesn't double-bind to depsgraph_update_post
+    via ``TokenCache`` instances created during their own ``install()``."""
     georeference_props = tool.Georeference.get_georeference_props()
     aggregate_props = tool.Aggregate.get_aggregate_props()
     nest_props = tool.Nest.get_nest_props()
@@ -427,23 +462,26 @@ def load_post(scene):
     NestDecorator.uninstall()
     WallAxisDecorator.uninstall()
     SlabDirectionDecorator.uninstall()
-    if georeference_props.should_visualise:
-        GeoreferenceDecorator.install(bpy.context)
-    if aggregate_props.aggregate_decorator:
-        AggregateDecorator.install(bpy.context)
-    if nest_props.nest_decorator:
-        NestDecorator.install(bpy.context)
-    if model_props.show_wall_axis:
-        WallAxisDecorator.install(bpy.context)
-    if model_props.show_slab_direction:
-        SlabDirectionDecorator.install(bpy.context)
-    if model_props.show_bounding_box:
-        BoundingBoxDecorator.install(bpy.context)
+    uninstall_decorator_cache_handlers()
+    try:
+        if georeference_props.should_visualise:
+            GeoreferenceDecorator.install(bpy.context)
+        if aggregate_props.aggregate_decorator:
+            AggregateDecorator.install(bpy.context)
+        if nest_props.nest_decorator:
+            NestDecorator.install(bpy.context)
+        if model_props.show_wall_axis:
+            WallAxisDecorator.install(bpy.context)
+        if model_props.show_slab_direction:
+            SlabDirectionDecorator.install(bpy.context)
+        if model_props.show_bounding_box:
+            BoundingBoxDecorator.install(bpy.context)
+    finally:
+        install_decorator_cache_handlers()
 
-    if preferences.should_use_snap and (scene := bpy.context.scene):
-        # Snapping is off by default in Blender, but in BIM, it's more useful to be on
-        scene.tool_settings.use_snap = True
-        # Match default Bonsai snaps
-        scene.tool_settings.snap_elements_base = {"EDGE", "EDGE_PERPENDICULAR", "VERTEX", "EDGE_MIDPOINT", "FACE"}
 
-    tool.Blender.sync_old_preferences()
+@persistent
+def load_post(scene):
+    _apply_save_file_invariants(scene)
+    _apply_user_preferences()
+    _install_viewport_overlays()