Document class-aware drug dialogue addition

Add curated class-aware drug dialogue layer to en-medical

Document the speech-register-per-domain design principle

Rebuild en-medical without general base: MTSamples 5x + synthetic dialogue 2x

Document layered-LM design and en-medical rebuild

Replace en-medical with layered build (general base + 3x medical, 3-gram)

Document en-medical

Add en-medical 4-gram (MTSamples + HealthCareMagic + PubMed abstracts)

Add en-general 4-gram LM

Add initial model card for Parakeet shallow-fusion KenLMs

initial commit