Module engine 

JP engine state machine — parallel in shape to inputx-wubi’s WubiEngine and inputx-pinyin’s segmenter so the composite layer can plug it in next to the existing engines without bespoke wiring.

State per session:

• buffer: raw ASCII romaji as the user types (preedit source)
• candidates: rebuilt on every keystroke, ordered hiragana → katakana → kanji-matching-current-on-yomi

No auto-commit / freq-tuning / pin learning — those are wubi/pinyin concerns and would only add ambiguity to a passthrough-style JP engine at this MVP stage.

Structs

Candidate

One JP candidate with its sub-source category, so the host UI can optionally render a hint (kana vs kanji) on top of the W/P/J source dot at the composite layer.

JapaneseEngine

JP engine. One per session. Cheap to construct — all data is in const tables in crate::romaji and crate::kanji.

Enums

KanaKind