Add BOSL2 compatibility helper and refactor rear crossmember

Introduced a stub for the BOSL2 `parent_modules()` function in lifetrac_v25_params.scad to suppress OpenSCAD warnings when BOSL2 is not present. Refactored rear_crossmember.scad to use 2D square extrusion for the plate, clarified bolt hole and lightening hole placement, and improved variable naming for readability and maintainability.

Author: dorkmo

LifeTrac-v25/mechanical_design/openscad/lifetrac_v25_params.scad

@@ -3,6 +3,13 @@
 // This file contains only the constants needed for part design
 // Full assembly uses lifetrac_v25.scad
 
+// -----------------------------------------------------------------------------
+// Compatibility helper
+// OpenSCAD throws warnings when the BOSL2 helper `parent_modules()` is missing.
+// We provide a stub that returns an empty list so part files can run cleanly
+// when opened directly without pulling in BOSL2.
+function parent_modules() = is_undef($parent_modules) ? [] : $parent_modules;
+
 // =============================================================================
 // MATERIAL CONSTANTS
 // =============================================================================

LifeTrac-v25/mechanical_design/parts/rear_crossmember.scad

@@ -7,28 +7,29 @@ include <../openscad/lifetrac_v25_params.scad>
 module rear_crossmember() {
     back_height = MACHINE_HEIGHT * 0.55;
     crossmember_width = TRACK_WIDTH + SANDWICH_SPACING + PANEL_THICKNESS * 2;
-    
-    // Create plate with holes for bolting to side panels
+    plate_thickness = PANEL_THICKNESS;
+
+    // Lay the plate flat in XY so projection captures all features
     difference() {
         // Main plate
-        cube([crossmember_width, PANEL_THICKNESS, back_height], center=true);
-        
-        // Bolt holes for mounting to side panels
-        // 4 holes per side (left and right)
+        linear_extrude(height=plate_thickness)
+        square([crossmember_width, back_height], center=true);
+
+        // Bolt holes for mounting to side panels (4 per side)
         for (side = [-1, 1]) {
             x_pos = side * (crossmember_width/2 - 50);
-            for (z = [back_height/4, back_height *3/4 - back_height/2]) {
-                translate([x_pos, 0, z])
-                rotate([90, 0, 0])
-                cylinder(d=BOLT_DIA_1_2 + 2, h=PANEL_THICKNESS+4, center=true, $fn=32);
+            for (z_pos = [back_height/4, back_height * 3/4]) {
+                y_pos = z_pos - back_height/2; // shift because square is centered
+                translate([x_pos, y_pos, plate_thickness/2])
+                cylinder(d=BOLT_DIA_1_2 + 2, h=plate_thickness+4, center=true, $fn=32);
             }
         }
-        
+
         // Lightening holes to reduce weight
         for (x = [-200, 0, 200]) {
-            translate([x, 0, back_height/2 - 100])
-            rotate([90, 0, 0])
-            cylinder(d=60, h=PANEL_THICKNESS+4, center=true, $fn=32);
+            y_pos = (back_height/2 - 100) - back_height/2; // place 100mm below top edge
+            translate([x, y_pos, plate_thickness/2])
+            cylinder(d=60, h=plate_thickness+4, center=true, $fn=32);
         }
     }
 }