Refactor depth surface handling and unify depth sampling rules

- Removed the `always_vertex` parameter from `build_surface_camera` to streamline depth sampling for cutouts and ensure consistent behavior across depth plane and panorama.
- Updated depth sampling logic to ensure that all free boundary points, including the original frame of the depth plane, use neighboring face depth instead of their own texture sampling.
- Adjusted the depth mask deletion process to operate on faces rather than points, eliminating the need for additional cleanup of dangling edges.
- Ensured that the cutout rules are consistently applied across all relevant nodes, removing exceptions and simplifying the logic.
- Updated tests to verify the new behavior, ensuring that cutout points follow the depth of adjacent faces and that the original frame maintains its depth sampling.
- Refactored geometry handling in multiple depth-related nodes to improve clarity and maintainability.

openspec/changes/refine-cutout-symmetry-seam/proposal.md

@@ -6,7 +6,7 @@
 
 - 对称节点的切口统一为「把越过对称面的顶点折叠到对称面，删除整体贴平的面与内部贴平边」，使零厚度与非零厚度输入的切边落在同一位置。
 - 补面只连接不落在对称面上的边界边。落在对称面上的切口由镜像后的合并直接焊成接缝，不再生成侧壁。
-- 新增 `Seam Smooth` 与重定义 `Fill Smooth`，都与上游 Boundary Smooth 同类：只移动面内分量、锁定对称轴，并按两圈渐变从切口环或界外轮廓向内部衰减。
+- 新增 `Seam Smooth` 并重定义 `Fill Smooth`：两者都只移动面内分量、锁定对称轴；`Seam Smooth` 按两圈渐变松弛切口环，`Fill Smooth` 按四圈渐变松弛 front 的衔接带，锚点为界外轮廓与对称面上的切缝，因此对称轴上的点同样参与。
 - 补面细分始终存在，不再以 `Fill Smooth > 0` 为开关；`Fill Smooth` 平滑界外轮廓，侧壁从平滑后的轮廓构建。
 - 不引入布尔、裁切算子、细分或新增采样参数。