US-5.2.2: Autocorrelation functions (VACF, end-to-end)

## User Story
> As an analyst, I want autocorrelation functions for velocity, dipole, end-to-end vector.

## Acceptance Criteria
- [ ] VACF -> vibrational spectrum
- [ ] End-to-end ACF -> Rouse/reptation relaxation time
- [ ] FFT for efficiency