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/records.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass
+class OperationRecord:
+    summary: str
+    detail: str
+    target_kind: str | None = None
+    target_id: int | None = None
+    target_logical_id: int | None = None
+    pick_position: tuple[float, float, float] | None = None
+    before_snapshot: dict[int, object] | None = None
+    after_snapshot: dict[int, object] | None = None
+    diff_stats: dict[str, object] | None = None