Add cross-frame tubes and bottom stiffener

Add support for 2x6 cross frame tubes and a bottom stiffener plate, including panel cutouts and mounting hardware. Changes include:

- Insert cross_tube_panel_cutout calls into left/right inner/outer side panels and add bottom-edge trim for inner panels.
- Add constants and helper functions for bottom plate, horizontal stiffener holes, and frame tube geometry/positioning (FRAME_TUBE_*, FRONT_FRAME_TUBE_Y, REAR_FRAME_TUBE_Y, etc.) and debug echo outputs.
- Implement horizontal_angle_iron_smart and frame_tube_angle_iron modules and place angle irons for mounting tubes to all wall panels (frame_tube_angle_irons).
- Add bottom_stiffener_plate module with matching hole patterns and horizontal angle-iron mounts; introduce cross_frame_tubes, cross_tube, and cross_tube_cutout_profile to model hollow 2x6 tubes and their mounts.
- Extend stiffener_side_panel_cutters to add holes for bottom-plate angles and for frame-tube angle mounts; remove prior mid-plate hole generation (mid stiffener plate simplified/removed).
- Comment out mid_stiffener_plate usage in base_frame and remove engine() call in assembly for simplification.

These changes add manufacturable CNC-friendly cutouts (rounded corners), bolt hole patterns for 3/8" and 1/2" bolts, and coordinate/placement logic to integrate cross tubes and bottom plate into the existing frame geometry.

Author: dorkmo