US-5.1.1: Radial distribution function g(r)

## User Story
> As an analyst, I want to compute the RDF from a trajectory.

## Acceptance Criteria
- [ ] Total and per-pair RDF
- [ ] PBC support (minimum image)
- [ ] Frame averaging
- [ ] Correct normalization (ideal gas density)

## New Crate
`crates/polysim-analysis/`