Adds 6 files in tests/synthetic/ exercising the determinism pipeline at the
SC7 brief floor (Jaccard >= 0.833). Plan fixture pair: 40 step titles each
with 38 shared (Jaccard 0.905). Review fixture pair: 30 finding-IDs each
with 28 shared (Jaccard 0.875). Reuses lib/parsers/jaccard.mjs +
lib/parsers/finding-id.mjs.
The new pair coexists with tests/lib/review-determinism.test.mjs which
holds the older SC4 (0.70) floor against tests/fixtures/ultrareview/.
The lower floor protects pipeline regressions; the higher floor anchors
the speedup brief's determinism aspiration.
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