#729: import STL 3D models (binary and ASCII)

[vangel76]

Fixes #729.
Part of ranked gap #13 "3D model formats and materials" (docs/gaps.md).

Problem

After Effects 26 imports STL models (usdStl) besides glTF and OBJ. We refused .stl files, and neither the Media Browser nor the import dialog listed them as models.

Fix

• New crates/model/src/stl.rs, a pure-Rust reader written from the public format description, with no new dependencies:
  • Binary: 80-byte header, u32 facet count, 50-byte facets. Bytes after the last facet are ignored.
  • ASCII: handles several solid blocks in one file and fan-triangulates loops with more than three vertices.
  • Format detection: an exact binary size wins over the solid keyword, because many binary headers also start with solid.
  • Colours: binary per-facet colours in both documented conventions (VisCAM/SolidView with bit 15 set; Materialise Magics COLOR= header with a part colour). Each colour becomes a plain dielectric material. Files without colour get no materials, so the renderer's default material applies, the same as OBJ without an MTL file.
  • Geometry: facets are flat-shaded, with the normal taken from the winding; the stored normal is used only when a facet is degenerate. Z-up (CAD/3D printing) is turned to the model's +Y-up with a proper rotation, so winding is kept.
  • Hostile input: truncated counts, empty files, files with no facets, non-finite coordinates and malformed ASCII vertices all return an error, never a panic. Allocation is bounded by the file size, and colour grouping uses a hash map, not a linear search.
• .stl is wired in everywhere models are recognised: effectcraft_model::format_of/load, MODEL_EXTENSIONS (import and probe, codec STL (…)), the Media Browser model kind, the File ▸ Import dialog filter, the system report import list and the layer.newModel help.
• docs/gaps.md row Unsaved changes: ask before closing a modified project; the modified mark follows undo #13 and docs/file-format-parity.md are updated (estimate unchanged).

Tests

• effectcraft-model:
  • stl_binary_is_faceted_and_turned_y_up: binary file whose header starts with solid, Y-up turn, winding, trailing bytes.
  • stl_ascii_with_polygon_loops_and_several_solids
  • stl_binary_colours: VisCAM and Magics.
  • stl_hostile_input_is_an_error
• effectcraft-media: stl_imports_as_a_model (is_importable plus probe as a model item). It fails without the change.

Gates

• cargo fmt --check: passed.
• cargo clippy -j 4 -p effectcraft-model -p effectcraft-media -p effectcraft-engine -p effectcraft-ui-egui --all-targets -- -D warnings: clean.
• Full test suites, all targets, 0 failures:
  • cargo test -j 4 -p effectcraft-model
  • cargo test -j 4 -p effectcraft-media
  • cargo test -j 4 -p effectcraft-engine (523 passed)
  • cargo test -j 4 -p effectcraft-ui-egui (499 passed)
• cargo xtask layers: ok.
• cargo check -p effectcraft-model -p effectcraft-media -p effectcraft-engine --target wasm32-unknown-unknown: ok.
• The full cargo xtask ci was not run locally because the machine is shared.

What remains

The rest of gap #13 is still open: FBX, USD/USDZ, and Substance 3D materials with mesh displacement. STL units are taken as-is, the same as OBJ, and scaled by Model Scale.

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