Bite-sized, TDD-style task breakdown for the first tier of the engine
redesign: Gaussian to natural-parameter storage, dense Vec storage
replacing HashMap, ScratchArena to eliminate per-event allocs,
Result-ifying the lone panic. No top-level public API change.
Acceptance gate: ≥3x speedup on Batch::iteration vs. baseline.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
