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Add opt-in compact speller target: make se.bhfst Packs the acceptor and the UN-determinised error-model union into divvunspell's native BHFST box format. The xre expansion step exists only so NFA-walking runtimes can search quickly; divvunspell with on-the-fly mutator determinisation searches the plain union at the same speed, so the expansion (and its hundreds of MB) can be skipped. Measured against the regular se.zhfst on the full typos corpus at the production config: every accuracy figure identical (top1 8881, top5 10115, 0 without suggestions), speed at parity, artifact 66 MB vs 131 MB. Not part of the default build and not a replacement: C++ hfst-ospell consumers and divvunspell releases without subset search need the regular se.zhfst.