Per-surface scanning is O(surfaces x copies x corpus). lv-mccarthy (108 surfaces,
36 copies) finished in 8 s; lv-hemingway (881 surfaces, 10 copies) was still running
at 5 minutes and had to be killed. A gate too slow to run is not a gate. Same trick
scan() already uses: build one alternation, map the matched string back to its
surface by stripping separators.
Regression: identical verdict and identical per-surface hit counts on the pre-fix
lv-mccarthy tree (5 surfaces, 78 hits) and on the fixed one (0). Re-derived on the
two shipped corpora with the committed instrument rather than a scratch probe:
lv-hemingway GATE FAILED Pasionaria, Primitivo, Chicote -- 6 hits each, all 6 copies
lv-bronte GATE PASSED 0
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