CMake configure:
1. Check for pre-compiled dicts/steno_dict.bin (or JAVELIN_DICT_BIN)
2. If missing, download Plover main.json from openstenoproject/plover
3. Run compile_dict.py to produce JSC4 binary
4. Embed via dict_embed.S
Downloaded JSON cached in dicts_src/ — persists across builds via
GH Actions west module cache. Only re-downloads if cache evicted.