Skip unsupported geometrically bounded surface representations

[Jassy930]

Fixes #24.

Summary

Some STEP exports include auxiliary
GEOMETRICALLY_BOUNDED_SURFACE_SHAPE_REPRESENTATION entities alongside supported
B-Rep representations.

Foxtrot can parse these entities, but the triangulation traversal previously
treated them as unknown root representations and panicked, aborting otherwise
usable solid geometry.

This change:

• explicitly recognizes
  Entity::GeometricallyBoundedSurfaceShapeRepresentation;
• logs the skipped entity ID;
• continues traversal without attempting to tessellate its curve geometry;
• adds a sanitized regression fixture and integration test.

Scope

This is a deliberately narrow fallback.

It does not implement tessellation for geometrically bounded surface
representations, and it does not replace the existing catch-all panic with a
blanket skip. Other unknown root representation types still fail as before.

The regression test verifies that:

• the baseline fixture does not contain the unsupported representation;
• the augmented fixture does contain and parse it;
• the baseline mesh is non-empty;
• adding the auxiliary representation does not change vertex or triangle counts.

Verification

Red/green regression test:

cargo test -p triangulate --test representation_fallback -- --nocapture

Before the source change, the new test panics when traversal reaches the
GeometricallyBoundedSurfaceShapeRepresentation. After the source change, it
passes.

Additional checks:

cargo test -p triangulate --features parallel --test representation_fallback
cargo test --workspace
rustfmt --edition 2018 --check triangulate/tests/representation_fallback.rs
git diff --check

All tests and checks pass with Rust and Cargo 1.96.0. Existing compiler and
future-compatibility warnings remain unchanged.