fix(howtogalaxy): should_simulate every dataset load; run under SMALL_DATASETS

[Jammy2211]

Slice 2/3 of the HowTo should_simulate migration (mirror of merged HowToLens#39). Migrate 17 raw exists-guards to al.util.dataset.should_simulate, add guard for tutorial_3_fitting's 2nd load, remove the dead howtogalaxy/+guides/ SMALL_DATASETS-unset overrides, regen notebooks. Verify all chapters pass on a fresh dataset/ at 16x16. Sims in scripts/simulators/ (plural). Prompt: PyAutoMind/active/howto_should_simulate_howtogalaxy.md.

[Jammy2211]

Shipped and merged in #30. All chapters green at 16x16 on fresh dataset. Slice 3/3 (HowToFit) next.