Bonsai patch: lossy-downgrade popup + per-recipe preset menu

Two new UX features in the IFC Patch panel, both backed by helpers on
bonsai.tool.Patch.

Lossy-downgrade confirmation popup. When the user picks the Migrate
recipe with a target schema older than the source's (IFC4 -> IFC2X3,
IFC4X3 -> IFC2X3), ExecuteIfcPatch.invoke shows a properties dialog
listing what's preserved vs lost: IfcIndexedPolyCurve flattened with
arcs approximated, IfcPolygonalFaceSet / IfcTriangulatedFaceSet
converted to IfcFacetedBrep, IFC4-only IfcElement subclasses (IfcLamp,
IfcPipeSegment, IfcGeographicElement, ...) demoted to
IfcBuildingElementProxy with the original class + PredefinedType
encoded into ObjectType, and PredefinedType enum values absent from
IFC2X3 dropped. The user explicitly approves before the recipe runs.

The popup is gated on tool.Patch.migration_is_lossy_downgrade() which
resolves the source schema via header-only parsing
(tool.Patch._patch_source_schema reads the first ~2KB and matches a
FILE_SCHEMA regex, then normalises via ifcopenshell.util.schema.
get_fallback_schema). Avoids a full ifcopenshell.open() on every
Execute click — multi-second saving on large files. The target schema
is looked up by argument name rather than position so it survives
recipe-parameter reordering.

Per-recipe preset menu. New BIM_MT_ifc_patch_presets + AddIfcPatchPreset
wire Blender's standard preset system into the panel. Each recipe gets
its own preset subdirectory (bonsai/ifc_patch/<RecipeName>/), so a
preset saved for ExtractElements does not pollute the Migrate preset
list. The preset operator uses Attribute.get_value_name() (single
source of truth for data_type -> storage-field mapping) to build the
preset_values list dynamically per recipe.

The recipe-change callback resets
BIM_MT_ifc_patch_presets.bl_label to the canonical title — Blender's
script.execute_preset mutates the menu's bl_label to the loaded
preset's name as a "currently-selected" indicator, and without an
explicit reset the previous recipe's preset name would falsely advertise
itself in the new recipe's menu.

tool.Patch gains get_preset_subdir, migration_is_lossy_downgrade,
_patch_source_schema as cross-cutting helpers. _SCHEMA_AGE module
constant provides the ordering used by the downgrade-detection
predicate.

Test coverage: 12 bim-lane tests under test/bim/module/patch/. The
truth table for migration_is_lossy_downgrade covers IFC4/IFC4X3 source
x downgrade/upgrade/same-schema target x Migrate/non-Migrate recipe.
The schema-sniffing tests write a real IFC4X3_ADD2 file to disk and
assert the helper resolves it to IFC4X3 (regression for the original
startswith iteration-order bug). An end-to-end test drives
bpy.ops.bim.execute_ifc_patch with an in-memory IfcLamp source and
verifies the on-disk IFC2X3 file contains a single
IfcBuildingElementProxy with ObjectType "IfcLamp/COMPACTFLUORESCENT"
and the original GlobalId preserved.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/tool/patch.py

@@ -18,18 +18,33 @@
 
 from __future__ import annotations
 
+import re
 from typing import TYPE_CHECKING, Any
 
 import bpy
 import ifcopenshell
+import ifcopenshell.util.schema
 import ifcpatch
 
 import bonsai.core.tool
+import bonsai.tool
 
 if TYPE_CHECKING:
     from bonsai.bim.module.patch.prop import BIMPatchProperties
 
 
+# Lower index = older schema. Used to detect downgrades vs upgrades.
+_SCHEMA_AGE = {"IFC2X3": 0, "IFC4": 1, "IFC4X3": 2}
+
+# Pretty-printed argument name for the ``Migrate`` recipe's schema parameter
+# (see UpdateIfcPatchArguments.pretty_arg_name in bim/module/patch/operator.py).
+_MIGRATE_SCHEMA_ARG_NAME = "Schema"
+
+# Match a STEP-encoded FILE_SCHEMA header: ``FILE_SCHEMA(('IFC4'));`` and the
+# IFC4X3_ADD2 / IFC2X3_TC1 variants. Captures the bare schema identifier.
+_IFC_FILE_SCHEMA_RE = re.compile(r"FILE_SCHEMA\s*\(\s*\(\s*'([^']+)'", re.IGNORECASE)
+
+
 class Patch(bonsai.core.tool.Patch):
     @classmethod
     def get_patch_props(cls) -> BIMPatchProperties:
@@ -54,6 +69,63 @@ class Patch(bonsai.core.tool.Patch):
             "SplitByBuildingStorey",
         )
 
+    @classmethod
+    def get_preset_subdir(cls) -> str:
+        """Resolve the preset subdirectory for the currently selected recipe.
+
+        Returns a stable string for the ``-`` placeholder so the menu and save
+        operator remain usable when no real recipe has been picked yet."""
+        recipe = cls.get_patch_props().ifc_patch_recipes or "-"
+        return f"bonsai/ifc_patch/{recipe}"
+
+    @classmethod
+    def migration_is_lossy_downgrade(cls) -> bool:
+        """``True`` when the currently configured patch is the ``Migrate``
+        recipe targeting an older schema than the input file. Used to gate
+        the destructive-migration confirmation dialog."""
+        props = cls.get_patch_props()
+        if props.ifc_patch_recipes != "Migrate":
+            return False
+        target_schema = next(
+            (arg.get_value() for arg in props.ifc_patch_args_attr if arg.name == _MIGRATE_SCHEMA_ARG_NAME),
+            None,
+        )
+        if not target_schema:
+            return False
+        source_schema = cls._patch_source_schema()
+        if not source_schema:
+            return False
+        return _SCHEMA_AGE.get(target_schema, -1) < _SCHEMA_AGE.get(source_schema, -1)
+
+    @classmethod
+    def _patch_source_schema(cls) -> str:
+        """Resolve the IFC schema of the configured input without parsing the
+        full file. For loaded-from-memory the schema is in the entity_instance
+        wrapper; for disk paths we read only the STEP file header (first ~2KB)
+        rather than ``ifcopenshell.open`` which parses the whole file."""
+        props = cls.get_patch_props()
+        if props.should_load_from_memory:
+            ifc_file = bonsai.tool.Ifc.get()
+            return ifc_file.schema if ifc_file else ""
+        if not props.ifc_patch_input:
+            return ""
+        try:
+            with open(props.ifc_patch_input, "rb") as f:
+                header = f.read(2048).decode("utf-8", errors="ignore")
+        except OSError:
+            return ""
+        match = _IFC_FILE_SCHEMA_RE.search(header)
+        if not match:
+            return ""
+        # Collapse IFC4X3_ADD2 / IFC2X3_TC1 / IFC4_ADD2 / IFC4X1 etc. to their
+        # base via the canonical normaliser — handles longest-prefix-first
+        # ordering correctly (IFC4X3 before IFC4) so we don't misclassify
+        # IFC4X3 files as IFC4.
+        try:
+            return ifcopenshell.util.schema.get_fallback_schema(match.group(1).upper())
+        except AssertionError:
+            return ""
+
     @classmethod
     def post_process_patch_arguments(cls, recipe: str, args: list[Any]) -> list[Any]:
         if recipe == "ExtractElements":