feat(engine): 支持 up_to_surface 非均匀 profile 裁剪拉伸

为 up_to_surface 终止条件新增裁剪语义，通过穿透拉伸与目标面布尔求交保留可达材料，修复非均匀相交时错误拒绝特征的问题。

backend/engine/cdsl_engine/build123d_adapter.py

@@ -5,7 +5,7 @@ from __future__ import annotations
 import math
 from typing import Any, Iterable
 
-from build123d import Axis, Edge, Face, Plane, Solid, Vector, Wire, export_step
+from build123d import Axis, Compound, Edge, Face, Plane, ShapeList, Solid, Vector, Wire, export_step
 
 from .runtime_types import AxisSpec, HoleSpec, PlaneSpec, TopologyRecord, Vector3, canonical_plane_signature
 
@@ -151,6 +151,43 @@ class Build123dGeometryAdapter:
         # 沿给定方向向量拉伸一个面，生成实体。
         return Solid.extrude(face, _vector(direction))
 
+    @staticmethod
+    def extrude_trimmed(face: Face, target: Any, direction: Vector3) -> Any:
+        """Extrude the profile to the target face, trimming unreached regions.
+
+        Issue #5: when a profile intersects the up_to_surface target
+        non-uniformly (part of the profile reaches the face, part hangs
+        outside it), a plain vector extrusion is wrong.  The CAD semantics is
+        to keep only the material between the profile and the target.  We
+        pierce the profile through the target, push the target face backward
+        by the same margin to build a slab, and keep their boolean common
+        (intersection) as the trimmed solid.
+        """
+        # 1. 采样点到目标的最远命中距离决定穿透余量；没有任何采样点命中
+        #    说明 profile 与目标面无交叠，无法裁剪（保留 extent_target_not_reached）。
+        unit = _vector(direction).normalized()
+        hits = [
+            Build123dGeometryAdapter._forward_intersection_distance(target, point, unit)
+            for point in Build123dGeometryAdapter.profile_sample_points(face)
+        ]
+        distances = [value for value in hits if value is not None]
+        if not distances:
+            raise ValueError("extent target is not reached by the profile")
+        margin = max(distances) + 2.0
+        # 2. 穿透拉伸 profile，同时把目标面向回推生成体层，二者求交即裁剪体。
+        #    build123d 的布尔交方法名是 intersect（不是 OCC 的 common），
+        #    且多实体结果返回 ShapeList，需要规整为单个 Solid / Compound。
+        pierced = Solid.extrude(face, unit * margin)
+        slab = Solid.extrude(target, -unit * margin)
+        trimmed = pierced.intersect(slab)
+        if isinstance(trimmed, ShapeList):
+            members = list(trimmed)
+            # build123d 类型桩未声明 make_compound，但运行时存在（宽泛类型桩噪音）。
+            trimmed = members[0] if len(members) == 1 else Compound.make_compound(members)  # pyright: ignore[reportAttributeAccessIssue]
+        if trimmed is None or (hasattr(trimmed, "is_empty") and trimmed.is_empty()):
+            raise ValueError("extent target produced an empty trimmed solid")
+        return trimmed
+
     @staticmethod
     def body_center(body: Any) -> Vector3:
         # 取主体包围盒的中心坐标，作为体心的近似。
@@ -236,8 +273,9 @@ class Build123dGeometryAdapter:
         return Solid.revolve(face, angle_deg, Build123dGeometryAdapter.axis(axis))
 
     @staticmethod
-    def fuse(body: Any | None, solid: Solid) -> Any:
+    def fuse(body: Any | None, solid: Any) -> Any:
         # 布尔并：没有既有主体时，直接以该实体作为新主体。
+        # 实参类型放宽为 Any：build123d 的布尔结果可能是 Solid 或 Compound。
         return solid if body is None else body.fuse(solid)
 
     @staticmethod