Base Flange
A base flange is the foundational flat plate of a sheet-metal body. It
extrudes a closed 2D profile by thickness and tags the result as a
sheet-metal wall, so subsequent flanges and bends can be added with full
parametric identity (preserved through Unfold).
You configure the project-level defaults — thickness, bend_radius,
k_factor — once via SheetMetalConfig and the helpers reuse them.