Building Yomu: furigana, tokenization, and offline dictionaries on iOS

This is the second part of a short series about building Yomu, an iOS app for reading Japanese text with adaptive furigana. In the first post, I described the reading problem that motivated the project. This one focuses on the technical core: how the app actually processes and renders Japanese text.
I am intentionally skipping large parts of the app here — UI, persistence, OCR, monetization. They are either standard iOS work or not particularly interesting in the context of Japanese text processing.