def make_phi_overlay(shape, phi):
    """Render PHI ONCE onto a canvas. Returns (overlay_array, overlays_meta)."""
    h, w = shape
    canvas = Image.new("L", (w, h), 0)  # blank canvas
    draw = ImageDraw.Draw(canvas)

    overlays, x, y = [], BURN_REGION_X, BURN_REGION_Y
    for entry in _phi_text_block(phi):
        x0, y0, x1, y1 = draw.textbbox((x, y), entry["line"], font=FONT)
        tw, th = x1 - x0, y1 - y0

        if x + tw > w or y + th > h:
            raise ValueError(
                f"rendered PHI overflows image: '{entry['line']}' "
                f"at ({x},{y}) size ({tw}x{th}), image {w}x{h}"
            )

        draw.text((x, y), entry["line"], font=FONT, fill=TEXT_COLOR)
        overlays.append({
            "phi_category": entry["phi_category"],
            "rendered_text": entry["line"],
            "phi_value": entry["value"],
            "bbox": [x, y, tw, th],
            "dicom_tag": entry["dicom_tag"],
        })
        y += th + LINE_GAP
    return np.array(canvas), overlays