feat(conversion): run anthy in-process through WebAssembly

Add a second engine for romaji-to-Japanese conversion that leaves the process
alone: anthy, compiled to a WebAssembly module and executed in the JVM by
Chicory's build-time AOT, so conversion needs no outbound request.

anthy/ carries the module sources, the WASI shims, the reproducible build
script, the committed AOT output and the dictionary (~23 MB, extracted into
the plugin data folder on first start; an existing file is left alone).
features.japaneseConversion.engine selects "google" or "anthy", and the anthy
block adds dictionary, workDirectory and sentenceLevel.

RomanjiConverter now sits on a KanjiConverter interface, so a whole message is
handed to the engine at once - anthy segments and reorders a sentence instead
of converting word by word - and the conversion cache is keyed by that
sentence. The engine tests drive the real module, the dictionary tests cover
the extraction, and everything else keeps running on the existing fakes.

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