Validated speedups vs Stan:
- H (hand-coded): ~9x (2 configs)
- A (autodiff): 3.8x-27.6x (33 configs)
- N (numerical): GP deferred (5 configs)

Features:
- Native HMC/NUTS with autodiff gradients
- ICAR/BYM2 spatial (13-22x faster than Stan)
- RW1/RW2/AR1 temporal (6-14x faster)
- ZI/Hurdle models (5.7-10.3x faster)
- benchmarks/ folder for reproducible validation

Known issues:
- GP spatial heisenbug (see feature/gp-heisenbug branch)