feat(core): add core engine primitives (M1)
Implements the 6-DOF rigid body dynamics core:
- Types (Vector6, Matrix6x6), Forces, BodyState value type
- ForceModel abstract interface (non-const compute, D-09)
- Body: mass matrix inversion at construction, friend CvodeIntegrator (D-11)
- CvodeIntegrator: BDF integrator via SUNDIALS CVODE 7.x, static rhsCallback
- Simulation: orchestrates Body + CvodeIntegrator + ForceModel registry
- Math utilities: LinearAlgebra (invert6x6, matVecProduct), AngleUtils, Interpolation
- 29 tests, 100% passing (zero-force, constant-force, SHO period, time accumulation)
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
