Add RTS or fixed-lag smoother for post-processed navigation solutions

[jbrodovsky]

Context

Offline smoothing (RTS/fixed-lag) is a standard tool for research and improves accuracy over filter-only solutions.

Scope / Requirements

• Implement RTS smoother (or fixed-lag smoother) for EKF/UKF/ESKF outputs.
• Store required filter history (state, covariance, gain) during the forward pass.
• Add CLI flag such as --smoother rts|fixed-lag and --smoother-lag <sec>.
• Output smoothed trajectory to a separate file or extra columns.

Acceptance Criteria

• Smoothed output is generated and improves RMS error vs filtered output on a test dataset.
• Results are deterministic with a fixed RNG seed.
• Documentation updated with a usage example.