Rust geometry core via pyo3 (v2 performance path)

[bamdadd]

Context

The analytic core is pure NumPy in src/multicam_sim/geometry.py: look_at_rotation (30), projection_matrix (60), project_point (69), segment_intersects_sphere (84), segment_intersects_aabb (105). For large scenes (many cameras × frames × points × occluders) this is the hot path. A v2 performance path is a good candidate for a compiled core.

What to do (larger, happy to mentor)

Port the geometry primitives to a Rust extension via pyo3 + maturin, exposed as an optional module that mirrors the geometry.py function signatures. Keep the NumPy implementation as the default and fallback; select the Rust path behind a flag / optional extra so the package still installs and runs pure-Python. Start with project_point and the two segment-intersection tests (the occlusion inner loop), which dominate.

Acceptance criteria

• A Rust project_point and segment_intersects_{sphere,aabb} return results identical to the NumPy versions within ~1e-9 across a randomized battery (parity test).
• The Rust path is opt-in; with it absent, everything works on the pure-Python core and all existing tests pass.
• A short benchmark (Rust vs NumPy) on a representative scene, posted on the issue or in docs/.
• Build wiring (maturin) documented; CI note for building the extension.

Pointers: geometry.py:30,60,69,84,105 (the port surface). Convention in DESIGN.md.