feat: 拆分 STEP 编辑器并完善最小系统

将原来的 main.py/step_model.py 拆分为 step_editor 包，补充测量、同域高亮、模型修复、历史导出、槽宽/凸台/边长等 MVP 编辑能力，并更新 README 和忽略规则。

step_editor/step_io.py

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+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+from OCC.Core.BRepBuilderAPI import BRepBuilderAPI_Transform
+from OCC.Core.IFSelect import IFSelect_RetDone
+from OCC.Core.Interface import Interface_Static
+from OCC.Core.STEPCAFControl import STEPCAFControl_Reader
+from OCC.Core.STEPControl import STEPControl_AsIs, STEPControl_Reader, STEPControl_Writer
+from OCC.Core.TDF import TDF_Label, TDF_LabelSequence
+from OCC.Core.TDocStd import TDocStd_Document
+from OCC.Core.TopLoc import TopLoc_Location
+from OCC.Core.TopoDS import TopoDS_Shape
+from OCC.Core.XCAFDoc import XCAFDoc_DocumentTool
+
+from .geometry_utils import _compound_from_shapes, _repair_shape, _unify_same_domain_shape
+from .model_types import PartNode
+
+
+def _load_with_xcaf(path: Path, product_names: list[str]) -> tuple[list[PartNode], TopoDS_Shape]:
+    doc = TDocStd_Document("pythonocc-step-document")
+    shape_tool = XCAFDoc_DocumentTool.ShapeTool(doc.Main())
+
+    reader = STEPCAFControl_Reader()
+    reader.SetColorMode(True)
+    reader.SetLayerMode(True)
+    reader.SetNameMode(True)
+    reader.SetMatMode(True)
+    reader.SetGDTMode(True)
+    status = reader.ReadFile(str(path))
+    if status != IFSelect_RetDone:
+        raise ValueError(f"Could not read STEP file: {path}")
+    if not reader.Transfer(doc):
+        raise ValueError(f"Could not transfer STEP document: {path}")
+
+    parts: list[PartNode] = []
+    free_shapes = TDF_LabelSequence()
+    shape_tool.GetFreeShapes(free_shapes)
+
+    def next_name(label: TDF_Label, index: int) -> str:
+        label_name = str(label.GetLabelName()).strip()
+        if label_name:
+            return label_name
+        if index - 1 < len(product_names):
+            return product_names[index - 1]
+        return f"Part {index}"
+
+    def add_node(
+        name: str,
+        kind: str,
+        shape: TopoDS_Shape,
+        parent_id: int | None,
+        depth: int,
+        path_text: str,
+    ) -> PartNode:
+        node = PartNode(len(parts) + 1, name, kind, shape, parent_id, depth, path_text)
+        parts.append(node)
+        return node
+
+    def transformed_shape(label: TDF_Label, locations: list[TopLoc_Location]) -> TopoDS_Shape:
+        shape = shape_tool.GetShape(label)
+        if shape.IsNull() or not locations:
+            return shape
+        location = TopLoc_Location()
+        for loc in locations:
+            location = location.Multiplied(loc)
+        return BRepBuilderAPI_Transform(shape, location.Transformation()).Shape()
+
+    def walk(label: TDF_Label, parent_id: int | None, depth: int, locations: list[TopLoc_Location], path_names: list[str]):
+        name = next_name(label, len(parts) + 1)
+        label_path = " / ".join(path_names + [name])
+
+        if shape_tool.IsAssembly(label):
+            node = add_node(name, "assembly", transformed_shape(label, locations), parent_id, depth, label_path)
+            components = TDF_LabelSequence()
+            shape_tool.GetComponents(label, components)
+            for i in range(1, components.Length() + 1):
+                component = components.Value(i)
+                if shape_tool.IsReference(component):
+                    referred = TDF_Label()
+                    shape_tool.GetReferredShape(component, referred)
+                    loc = shape_tool.GetLocation(component)
+                    walk(referred, node.id, depth + 1, locations + [loc], path_names + [name])
+                else:
+                    walk(component, node.id, depth + 1, locations, path_names + [name])
+            return
+
+        if shape_tool.IsSimpleShape(label) or shape_tool.IsShape(label):
+            add_node(name, "part", transformed_shape(label, locations), parent_id, depth, label_path)
+
+    for i in range(1, free_shapes.Length() + 1):
+        walk(free_shapes.Value(i), None, 0, [], [])
+
+    display_shapes = [p.shape for p in parts if p.kind == "part" and not p.shape.IsNull()]
+    if not display_shapes:
+        display_shapes = [p.shape for p in parts if not p.shape.IsNull()]
+    return parts, _compound_from_shapes(display_shapes)
+
+
+def _load_plain_step(path: Path) -> TopoDS_Shape:
+    reader = STEPControl_Reader()
+    status = reader.ReadFile(str(path))
+    if status != IFSelect_RetDone:
+        raise ValueError(f"Could not read STEP file: {path}")
+    if not reader.TransferRoots():
+        raise ValueError(f"Could not transfer STEP roots: {path}")
+    return reader.Shape()
+
+
+def _parse_product_names(path: Path) -> list[str]:
+    text = path.read_text(errors="ignore")
+    names = re.findall(r"PRODUCT\('((?:''|[^'])*)'", text)
+    return [name.replace("''", "'") for name in names if name.strip()]
+
+
+def _write_step(shape: TopoDS_Shape, filename: Path) -> None:
+    if shape.IsNull():
+        raise ValueError("Cannot export a null shape.")
+    filename.parent.mkdir(parents=True, exist_ok=True)
+    Interface_Static.SetCVal("write.step.schema", "AP214IS")
+    writer = STEPControl_Writer()
+    writer.Transfer(shape, STEPControl_AsIs)
+    status = writer.Write(str(filename))
+    if status != IFSelect_RetDone:
+        raise IOError(f"Could not write STEP file: {filename}")
+
+
+def _prepare_shape_for_step_export(shape: TopoDS_Shape) -> TopoDS_Shape:
+    if shape.IsNull():
+        return shape
+    try:
+        repaired = _repair_shape(shape)
+        unified = _unify_same_domain_shape(repaired)
+        repaired_unified = _repair_shape(unified)
+        if repaired_unified.IsNull():
+            return shape
+        return repaired_unified
+    except Exception:
+        return shape