chore: stop tracking simulator-regenerated datasets (#8)

The dataset/imaging/howtolens fits files are regenerated at runtime
by tutorials in scripts/howtolens/chapter_1_introduction (which call
SimulatorImaging when the dataset folder is missing). They were
already covered by the dataset/ rule in .gitignore but had been
committed before the rule existed, so git kept tracking them and
test runs left them perpetually dirty.

Co-authored-by: Jammy2211 <[email protected]>
Co-authored-by: Claude Opus 4.7 (1M context) <[email protected]>

Author: 3 people