v0.2.1 marks the completion of the major refactor and diagnostic phase.

Key outcomes:

• Clean separation of Stage 1 (collapsed HMM), Stage 2 (kinematics + cameras), and Stage 3 (directional HMM prior)
• Numerical stability and gradient correctness verified
• Sampling pathologies identified and localized (camera projection, root/direction/length redundancy)

v0.2.1 is architecturally complete but not sampling-robust in all regimes.

Development now moves to v0.2.2, focused on data-informed priors, stabilization, and scalability.