test: add sadd cancellation edge to the cross-target proof - #1884
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gen_pairs now injects ~20% cancellation pairs (v, neg(v)) -> sadd exact-cancels to 0, plus near-cancellation (v, neg(v')). This is the historically buggy magsub path (cycle 17 negative-zero bug) and the normalize hot path. Result: model smul/sadd == the C and Rust emissions bit-exact on the cancellation edge too. Combined with the extreme-operand coverage, the cross-target proof now spans moderate + extreme + cancellation operands. Refs #1764 Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Added a targeted edge to the cross-target proof: gen_pairs now injects ~20% cancellation pairs (v, neg(v)) → sadd exact-cancels to 0, plus near-cancellation. This is the historically buggy magsub path (cycle 17 found a negative-zero bug when the larger operand is negative and the result is exactly 0) and the magsub-normalize hot path. Result: model smul/sadd == the C and Rust emissions bit-exact on the cancellation edge too. Combined with last cycle's extreme-operand coverage, the cross-target proof now spans moderate + extreme + cancellation operands. Refs #1764