Add dual-material interface layers, multi-STL stacks, free-form layer heights

[SchoepsLabs]

I've been using MultiStack on a Bambu H2D but couldn't get reliable separation with the air gap + ironing approach at 0.24/0.3 mm layer heights. This PR adds a dual-material interface mode plus several robustness fixes, all print-tested. Everything keeps the single-file, no-dependency design (earcut is inlined, MIT).

Interface layer mode (filament 2)

New "Separation mode" toggle: Air gap or Interface layer. Interface mode inserts thin slabs between tiles, assigned to extruder 2. With a non-bonding pairing (PLA interface between PETG tiles, or PETG between PLA) stacks pop apart cleanly with no fusing and no ironing tricks.

• The interface footprint is a true cross-section of the tile at 45% of its height (plane/mesh intersection -> closed contours with holes -> extruded watertight prism). A naive Z-squash of the tile mesh produces interfaces with missing interiors, because the slice plane can land where the tile cross-section is only rims/peaks.
• • Defaults follow Bambu's proven support-interface recipe: 2 solid interface layers, direct contact, with an optional release gap (air layers above each interface, like support top Z distance) for stickier pairings.
• • Interface parts are pre-configured via per-part settings: extruder 2, ironing off, 80 mm/s (Bambu's stock support-interface speed).

Everything is snapped to the slicing grid

The user enters their layer height; gaps and interfaces are specified in whole layers and placed exactly on layer boundaries (a sub-layer pad absorbs tile heights that aren't layer-height multiples). Without this, at 0.24/0.3 mm layer heights the old fixed 0.20 mm gap lands mid-layer and tiles fuse, and interfaces partially vanish from the sliced output.

Native multi-part assemblies

Each stack is emitted as a single 3MF object with tiles/interfaces as components at fixed relative positions, with per-part settings in model_settings.config. This removes the "multi-part object detected" dialog on import and fixes a nasty failure where Bambu Studio dropped the interface object to the build plate (shifting every interface down one boundary and causing gcode-conflict warnings).

Multi-STL stacks

Upload several STLs (tiles, corners, edges) in one job; each gets its own stack height. Stacks are shelf-arranged with 12 mm spacing and centered, with a log warning if the layout exceeds 256 mm.

Smaller changes

• Layer height is a free-form input (0.20 / 0.24 / 0.30...), max stack height 20 -> 45
• • Per-object/part extruder assignments written to model_settings.config

Tested

• Headless generation tests for both modes: placement math, per-part extruder assignment, interface mesh watertightness/winding/volume validated with trimesh against geometry with through-holes, cylindrical bores, and top bosses; assembled 3MF round-tripped through an independent parser
• • Real prints on a Bambu H2D (dual nozzle) at 0.24 and 0.3 mm: PLA tiles/PETG interfaces and PETG tiles/PLA interfaces - clean separation, no fusing
    Happy to split features into separate PRs if you'd rather take them piecemeal.