Building trajectories

Centered on how to build trajectories to generate proposals for algorithms in the HMC family:

- Integrate forward in time for a fixed number of steps + momentum inversion
- Alternate between integrating forward and backward in time
- Same but with the number of steps increasing geometrically

No file or test is named; start by locating the sampling-book tutorial source covering HMC trajectories and reviewing how related material is organized. Cover fixed-step integration with momentum inversion, alternating forward and backward integration, and geometrically increasing step counts. Done means all three trajectory approaches are documented clearly for proposal generation.

Written by the indexing model from the issue text.