bare/gpt-5.4-mini / tier-3-butt-hinge

```openscad
$fn = 96;

// Parameters
leaf_len = 30;
leaf_w   = 25;
leaf_th  = 2;

pin_d = 4;
pin_len = 32;

knuckle_od = 8;
knuckle_id  = 4.6;
knuckle_len = 6;

clear_x = 0.05;   // tiny assembly clearance
clear_z = 0.02;

m3_through_d = 3.2;
m3_csk_top_d = 6;
m3_csk_depth = 1;

leaf_hole_y = [-8, 0, 8];   // 8 mm pitch, 3 holes

// Derived positions
y_min = -leaf_len/2;
y_max =  leaf_len/2;

module leaf_base(side=1) {
    // side: -1 left, +1 right
    translate([side*(pin_len/2 + leaf_w/2 - clear_x), 0, 0])
        cube([leaf_w, leaf_len, leaf_th], center=true);
}

module knuckle_segment(yc) {
    translate([0, yc, 0])
    difference() {
        cylinder(d=knuckle_od, h=knuckle_len, center=true);
        cylinder(d=knuckle_id, h=knuckle_len + 2, center=true);
    }
}

module pin_axis() {
    translate([0, 0, 0])
        cylinder(d=pin_d, h=pin_len, center=true);
}

module countersunk_holes(side=1) {
    // side: -1 left leaf, +1 right leaf
    // holes are on the side far from knuckles
    x_hole = side*(pin_len/2 + leaf_w - 6);  // near outer edge, inside plate
    for (yy = leaf_hole_y) {
        translate([x_hole, yy, 0]) {
            // through hole
            translate([0, 0, -leaf_th/2 - 0.1])
                cylinder(d=m3_through_d, h=leaf_th + 0.2, center=false);
            // countersink from outer surface
            translate([0, 0, leaf_th/2 - m3_csk_depth])
                cylinder(d1=m3_csk_top_d, d2=m3_through_d, h=m3_csk_depth, center=false);
        }
    }
}

module leaf_with_knuckles(side=1) {
    // side: -1 left leaf, +1 right leaf
    difference() {
        union() {
            // plate
            leaf_base(side);

            // knuckles: left leaf gets 3 segments, right leaf gets 2 segments
            if (side < 0) {
                // outer, middle, outer
                knuckle_segment(-9);
                knuckle_segment(0);
                knuckle_segment(9);
            } else {
                // two middle segments
                knuckle_segment(-4.5);
                knuckle_segment(4.5);
            }
        }
        countersunk_holes(side);
    }
}

// Assemble opened state (180°): both leaf flats on same plane
// Pin axis is along Y and centered at X=0, Z=0.
union() {
    // Left leaf extends to -X
    translate([-(pin_len/2 + leaf_w/2), 0, 0])
        rotate([0, 180, 0])
            leaf_with_knuckles(-1);

    // Right leaf extends to +X
    translate([ (pin_len/2 + leaf_w/2), 0, 0])
        leaf_with_knuckles(+1);

    // Pin axis
    pin_axis();
}
```