bare-low/gpt-5-mini / tier-2-hex-bolt

```openscad
// M8 hex bolt (simplified: no threads, cylindrical shank)
// Head: hexagonal prism, across-flats = 13 mm, height = 5.3 mm
// Shank: cylinder, diameter = 8 mm, length = 30 mm
// Head bottom at z = 0, shank extends to z < 0, central axis along +Z

$fn = 64; // smooth cylinder

module hex_head(across_flats = 13, height = 5.3) {
    // For a regular hexagon: across_flats = R * sqrt(3)  => circumradius R = across_flats / sqrt(3)
    R = across_flats / sqrt(3);
    // Create a 6-sided regular polygon with circumradius R, centered at origin
    linear_extrude(height = height, center = false, convexity = 10)
        polygon(points = [ for(i = [0:5]) let(a = i*360/6) [ R*cos(a), R*sin(a) ] ]);
}

module shank(diameter = 8, length = 30) {
    // Cylinder with top at z = 0, extending to negative z
    translate([0,0,-length]) // move base to z = -length
        translate([0,0,0]) // ensures top at z=0 after cylinder creation
            cylinder(h = length, d = diameter, center = false);
}

// Assemble: head from z=0 up, shank from z=0 down
union() {
    // Head: bottom at z = 0
    translate([0,0,0]) hex_head(13, 5.3);
    // Shank: top at z = 0, extends to z = -30
    shank(8, 30);
}

// Optional: create a module and call it (already assembled above)
// bolt();
```