Add typeset_prose splits and augmentation improvements
Adds a prose-only synthetic split (typeset_prose_{train,val,test}) rendered
in handwriting-style fonts with no math delimiters, addressing the prior bias
P(Math | Handwritten) ≈ 1 learned from math-only real handwriting datasets.
Also extends _augment() with ElasticTransform (baseline wobble) and
RandomPerspective (photographed-paper effect), plus optional ruled-line
overlay for notebook paper simulation.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>