Summary

Two patch-module improvements that landed together because they share the same Bonsai panel:

1. Defensive IFC4 / IFC4X3 → IFC2X3 downgrade in the Migrate ifcpatch recipe. Previously crashed mid-loop with the cryptic RuntimeError: Entity with name '' not found in schema 'IFC2X3' — class_4_to_2x3.json marks IFC4-only classes with an empty-string sentinel and the old code forwarded that to create_entity. Real files routinely contain IfcPolygonalFaceSet, IfcTriangulatedFaceSet, IfcIndexedPolyCurve, IfcLamp, IfcPipeSegment, IfcGeographicElement, etc.
2. Per-recipe preset system for the IFC Patch panel. Save / load / remove sets of recipe arguments via Blender's standard preset infrastructure, scoped per recipe so an ExtractElements preset doesn't pollute the Migrate preset list. Persists across files and sessions via the standard Blender preset directory.

User-facing features (Bonsai panel)

Lossy-downgrade confirmation popup

ExecuteIfcPatch.invoke now shows a confirmation dialog before running a destructive migration. Fires when the user picks Migrate with a target schema older than the source's (IFC4 → IFC2X3, IFC4X3 → IFC2X3). Lists what's preserved (geometry via curve flatten + face-set → brep conversion) vs lost (IFC4-only element classes demoted to IfcBuildingElementProxy, PredefinedType enum values absent from IFC2X3 dropped). User explicitly approves before the recipe runs.

Backed by tool.Patch.migration_is_lossy_downgrade(), which resolves the source schema via header-only parsing (regex on first ~2KB) + get_fallback_schema normalisation — avoids a full ifcopenshell.open() on every Execute click. The target schema is looked up by argument name, not position.

Per-recipe preset system

New BIM_MT_ifc_patch_presets menu + AddIfcPatchPreset operator wire Blender's standard preset infrastructure into the IFC Patch panel. The preset row sits above the dynamic argument fields with a "Preset" heading, the menu dropdown, and + / - buttons.

• Per-recipe preset subdirectory (bonsai/ifc_patch/<RecipeName>/) — presets created for ExtractElements don't show up when Migrate is selected and vice versa.
• Preset save uses Attribute.get_value_name() (single source of truth for data_type → storage-field mapping) to build the preset_values list dynamically per recipe — works for every recipe out of the box.
• 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; without an explicit reset the previous recipe's preset name would falsely advertise itself in the new recipe's menu.
• Standard Blender preset persistence — presets live under ~/.../scripts/presets/bonsai/ifc_patch/<RecipeName>/<preset_name>.py and survive across files and sessions.

What changed under the hood

Library layer (ifcopenshell.util):

• schema.Migrator(fallback_element_to_proxy=False) opt-in — IFC4-only IfcElement subclasses migrate to IfcBuildingElementProxy instead of raising. Default preserves the strict failure-on-unmappable contract for existing callers (classification API).
• schema.geometry_classes_introduced_after(target, source) — derives the IfcRepresentationItem subclasses present in source but absent in target directly from the loaded schemas (cached). Replaces hand-curated class lists. ifc4_only_geometry_classes() retained as an alias.
• schema.generate_default_value — synthesises a unit IfcAxis2Placement2D / IfcAxis2Placement3D when downgrading entities whose Position became required (IfcIShapeProfileDef etc.).
• schema.migrate_attribute — structural enum-mismatch detection via get_enum_items replaces the previous string-matched RuntimeError catch, so upgrade paths still surface real bugs.
• shape_builder.polygonal_face_set_to_faceted_brep(face_set) — direct entity-level conversion of IfcPolygonalFaceSet / IfcTriangulatedFaceSet to IfcFacetedBrep, preserving topology incl. IfcIndexedPolygonalFaceWithVoids inner bounds.
• shape_builder.arc_to_polyline_points(start, mid, end, n) — circle-fit chord approximation for arc segments.

Recipe layer (ifcpatch.recipes.Migrate + DowngradeIndexedPolyCurve):

• New preprocessing pipeline gated on target=IFC2X3 + source in (IFC4, IFC4X3): curve flatten → face-set → brep conversion → orphan purge.
• DowngradeIndexedPolyCurve now handles IfcArcIndex segments (16-chord approximation) and absent Segments list (IFC4 implicit-polyline case). Multi-index IfcLineIndex handled correctly.
• Post-pass encodes <OriginalClass>/<PredefinedType> into ObjectType when empty, so the lost subclass identity survives the downgrade as searchable text (e.g. "IfcLamp/COMPACTFLUORESCENT").
• Migration loop collects per-entity failures into a list; summary RuntimeError fires at end naming up to 20 with their inverse references — no more cryptic mid-loop crash.

Commits (single-concern per AGENTS.md)

1. a2dafc9ce — ifcopenshell.util: schema-aware downgrade helpers
2. f710929e9 — ifcpatch Migrate: defensive IFC4/IFC4X3 → IFC2X3 downgrade
3. 2ab5ca922 — ifcpatch: small recipe polish (ExtractElements docstring, FixArchiCADToRevitDoorSwings null-check)
4. 44c0c2916 — Bonsai patch: lossy-downgrade popup + per-recipe preset menu

Test coverage

Plus verified untouched: 1426/1426 test/api/ pass (the other Migrator callers — add_classification, add_reference — keep the strict-default contract).

The 12 new bim-lane tests under test/bim/module/patch/ cover:

• Lossy-downgrade detection truth table (6 cases: source × target × recipe)
• Header-only schema parsing incl. IFC4X3_ADD2 regression pin
• Preset menu label reset on recipe change
• End-to-end downgrade: in-memory IFC4 IfcLamp → bpy.ops.bim.execute_ifc_patch → on-disk IFC2X3 file with IfcBuildingElementProxy carrying ObjectType="IfcLamp/COMPACTFLUORESCENT" and the original GlobalId.

Verification

• Lint clean across all touched files (black --check, ruff check).
• Original failing IFC4 file reported by the issue exports to IFC2X3 successfully (manually verified in Bonsai).
• Regression pins for the two highest-risk bugs caught during review: IFC4X3_ADD2 schema-prefix collision (the startswith("IFC4") mismatch) and the IFC4X3-source preprocessing gap.

API additions / changes

• Migrator(*, fallback_element_to_proxy: bool = False) — new keyword-only constructor parameter. Default preserves backwards compat.
• ifcopenshell.util.schema.geometry_classes_introduced_after(target, source="IFC4") — new public function.
• ifcopenshell.util.shape_builder.polygonal_face_set_to_faceted_brep(face_set) — new public function.
• ifcopenshell.util.shape_builder.arc_to_polyline_points(start, mid, end, subdivisions=16) — new public function.
• bonsai.tool.Patch.{get_preset_subdir, migration_is_lossy_downgrade} — new public class methods.
• Migrator no longer raises a bare RuntimeError: Entity with name '' not found for unmappable IFC4-only classes — instead raises NotImplementedError with the failing class + inverse references named, or falls back to IfcBuildingElementProxy when the opt-in flag is set.

Known limitations / out of scope

• IFC4X3-only entities outside any IfcProduct representation (e.g. orphan IfcAlignmentCurve in standalone-utility files) still surface in the failure summary rather than being auto-converted. These are rare in real models.
• The Migrate recipe still raises RuntimeError for non-IfcElement IFC4-only classes (rels, materials, time entities). Same as before — these have no meaningful generic IFC2X3 stand-in.
• Preset persistence is per-Blender-version (standard Blender preset directory is version-specific). A future Blender upgrade does not migrate presets automatically.

AI-assisted code attribution (per AGENTS.md §0b)

All four commits in this PR were authored with the assistance of an AI coding tool (commit bodies carry the marker). New files added by this PR include the top-of-file # This file was generated with the assistance of an AI coding tool. marker. The architectural decisions (API shapes, opt-in flag, ObjectType encoding strategy, per-recipe preset subdirectory) were reviewed and approved by the human author before each commit.