Phase space mapping enhancements

Ideas:
1. Using GC tracing for Liouville mapping
2. Adaptive Phase Space Gridding
3. Particle Splitting and Russian Roulette (i.e. Importance Sampling with Trajectory Splitting and Merging)

The issue lists three broad directions—GC tracing for Liouville mapping, adaptive phase-space gridding, and particle splitting or Russian roulette—but names no files, tests, or entry points. First map the current phase-space mapping and particle-tracing implementation, then select and scope one direction with a measurable validation target; done requires an agreed design and working implementation.

Written by the indexing model from the issue text.