iter-png-2/gpt-5 / tier-2-hex-bolt

```openscad
// Parameters
af = 13;           // across-flats for hex head (mm)
head_h = 5.3;      // head height (mm)
shank_d = 8;       // shank diameter (mm)
shank_len = 30;    // shank length (mm)

// Derived
head_r = af / sqrt(3);  // circumradius for hex to achieve given across-flats

// Hex head (base at z=0, extends to +Z)
module hex_head(h, r) {
    cylinder(h=h, r=r, $fn=6);
}

// Shank (extends from z=0 down to -Z)
module shank(len, d) {
    translate([0,0,-len])
        cylinder(h=len, r=d/2, $fn=96);
}

// Assemble
union() {
    hex_head(head_h, head_r);
    shank(shank_len, shank_d);
}
```