Custom handwriting model

Create a more fully-fledged custom handwriting model targeted towards handwriting of Chinese learners (ideas: identify errors, display handwriting suggestions, custom metrics for correctness)

The issue names no files, tests, or entry points. First define the model scope and success criteria for Chinese learners; the work is done when handwriting errors, suggestions, and correctness metrics are implemented and validated against agreed requirements.

Written by the indexing model from the issue text.