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>