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/workers.py

@@ -0,0 +1,51 @@
+from __future__ import annotations
+
+from PySide6.QtCore import QObject, Signal, Slot
+
+
+class EditWorker(QObject):
+    finished = Signal(object)
+    failed = Signal(str)
+
+    def __init__(self, action):
+        super().__init__()
+        self.action = action
+
+    @Slot()
+    def run(self) -> None:
+        try:
+            self.finished.emit(self.action())
+        except Exception as exc:
+            self.failed.emit(str(exc))
+
+
+class ScanWorker(QObject):
+    finished = Signal(object)
+    failed = Signal(str)
+
+    def __init__(self, action):
+        super().__init__()
+        self.action = action
+
+    @Slot()
+    def run(self) -> None:
+        try:
+            self.finished.emit(self.action())
+        except Exception as exc:
+            self.failed.emit(str(exc))
+
+
+class LoadWorker(QObject):
+    finished = Signal(object)
+    failed = Signal(str)
+
+    def __init__(self, action):
+        super().__init__()
+        self.action = action
+
+    @Slot()
+    def run(self) -> None:
+        try:
+            self.finished.emit(self.action())
+        except Exception as exc:
+            self.failed.emit(str(exc))