Two unrelated functions shared the name assert_corpus_sound: the
practice-corpus soundness check in evals/deduction_serve/practice/gold.py
(no args) and the curriculum lane's case-contract check in
evals/curriculum_serve/runner.py (domain, cases). Renamed to
assert_practice_gold_sound and assert_lane_cases_sound respectively so
each name states what it validates, and updated all call sites, the
gold.py __all__ export, and two prose mentions that named the function.
[Verification]: uv run --python 3.12.13 core test --suite smoke -q → 569
passed; --suite deductive -q → 285 passed.
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