compose_multicolor_3mf

Creates a **single .3mf, ready to slice**, containing all parts with
per-part AMS/extruder slot assignments.  This is the correct way to hand
a multi-color design to a slicer — it receives one file, not multiple.
It is a model, not a sliced plate, so no printer starts it as it stands:
slice it first.  ``fits_bed`` says whether the plate fits the bed it was
composed for; when it does not, the message says by how much and what to
do, and does not offer it for printing.

Parts whose bodies touch or overlap are one print: they become ONE
object with a part per input, and where two overlap the smaller part is
printed.  Parts placed apart stay separate objects.

Compatible with:
* **BambuStudio / OrcaSlicer / Bambu A1, X1, P1 + AMS** — read ``Metadata/model_settings.config``
* **PrusaSlicer / MMU** — reads ``Metadata/Slic3r_PE_model.config``
* **Cura** and any 3MF-capable slicer

Typical two-color workflow::

    # 1. Export body STL (main color, e.g. grey PLA)
    # 2. Export accent STL (second color, same coordinate origin)
    # 3. Compose:
    result = compose_multicolor_3mf(parts=[
        {"stl_path": "/tmp/body.stl",    "extruder": 1,
         "name": "body",    "color": "#AAAAAA", "material": "PLA Grey"},
        {"stl_path": "/tmp/qr_pads.stl", "extruder": 2,
         "name": "qr_code", "color": "#111111", "material": "PLA Black"},
    ])
    # 4. Slice and print (or slice_model to look at the slice first):
    slice_and_print(result["output_path"])

Args:
    parts: List of part dicts.  Each dict requires:

        * ``stl_path`` (str) — absolute path to the STL for this part
        * ``extruder`` (int) — 1-indexed AMS slot (1 = AMS tray 1 on
          Bambu); omitted, 1

        Optional per-part keys:

        * ``name`` (str) — label shown in the slicer object list
        * ``color`` (str) — hex preview color e.g. ``"#AAAAAA"`` (display only)
        * ``material`` (str) — filament label e.g. ``"PLA Grey"`` (display only)

    output_path: Where to write the .3mf.  Defaults to a temp file whose
        path is returned in the result.
    plate_width: Print plate X dimension in mm (default 256 for legacy
        callers without a printer id).
    plate_depth: Print plate Y dimension in mm (default 256).
    material: Optional material the parts will be printed in (e.g.
        ``"PETG"``).  A spool on record in a clearly different
        material is then not named as one to load.
    printer_id: Optional supported printer model id.  When it resolves,
        its build volume overrides ``plate_width`` / ``plate_depth``.
        The composer centres an off-plate group on THIS plate, so name
        the printer whenever you know it — a group arranged for a
        larger or smaller bed and composed against the 256 default is
        re-centred onto a plate the machine does not have.

Returns:
    Dict with ``success``, ``output_path``, ``parts``, ``objects`` (build
    objects, after touching parts are joined), ``colours`` (extruder
    slots used), ``total_triangles``, ``total_vertices``, ``extruder_map``
    (which parts print from each slot), ``fits_bed`` (``False`` when the
    plate is bigger than its bed, ``None`` when it could not be measured),
    ``bed_fit`` (the sizes behind that verdict), and ``message``.  It also says
    whether the part colours are on the printer, as
    ``colour_availability`` (absent when Kiln cannot see what is loaded).
    Relay its ``say`` as written and add nothing about filament: never
    suggest buying filament unless the person asks for one or says yes
    to the offer in ``say``.  On a yes, call ``find_closest_filaments``
    with these colours and the print's material.