3D-printing support: print-ready export + validation

[krazyjakee]

Background

MoGen's geometry is already fundamentally print-friendly: CSG output is watertight by construction (Manifold + clean_csg_output weld/degenerate-cull in crates/mogen-geom/src/cleanup.rs), and length units normalize to metres at parse time with mm/cm/m/km/in/ft/yd suffixes (crates/mogen-dsl/src/parser.rs), so millimetre-accurate authoring works today.

But the pipeline stops short of guaranteeing or producing a print-ready file. This issue tracks the optional features that would close that gap.

Gaps

• Open primitives — plane, disc, curved_plane, spline_ribbon are non-manifold by design (fine for game assets, unprintable as a solid).
• No print export format — GLB/FBX only; STL/3MF require external conversion.
• No manifold validation — validate_graph covers skinning/connectivity topology but never verifies the final mesh is watertight/manifold. is_closed_manifold() / is_csg_manifold() (crates/mogen-geom/src/csg.rs:146) exist but only gate the merge pass.
• No wall-thickness check — thin features below a printer's nozzle minimum silently fail.
• No self-intersection detection — Manifold avoids it for CSG, but pre-CSG sibling meshes can still overlap.

Proposed features (rough priority order)

1. STL / 3MF exporter in mogen-export. STL is trivial (triangle soup); 3MF is the modern choice (units, colour, multi-object). Biggest single unlock.
2. mogen check --print validation mode — surface non-manifold edges (edge shared by ≠2 tris), open boundaries, zero-area tris, and disconnected shells as diagnostics. Reuses existing manifold helpers instead of only gating merge.
3. Minimum wall-thickness check — warn on thin features given a target nozzle diameter (SDF/ray-sampling heuristic).
4. Print export mode that forces a single watertight shell — extend the existing merge_sibling_meshes CSG-union pass across the whole tree rather than scoped to same-material siblings.
5. Open-primitive guard — error early when a print target uses plane/disc/etc. as visible surfaces.

Suggested first step

Feature (2) reuses existing diagnostic infrastructure and manifold helpers, and tells users why a model won't print before investing in the exporter (1).