Replaces the synthetic ~5.5-node/msg pushes with a deterministic generator
(scripts/fixture.ts): lorem-ipsum user turns + fat assistant turns (markdown +
reasoning + 1-15 tool parts with multi-line results) driven through the real
apply()/commitSnapshot paths. mem-bench.tsx pumps it + checks the resume path.
Realistic cost is ~20.4 renderables/msg (3.7x synthetic); informed the cap tune.
