AlgoScan

TestSelectF128BoundaryStraddle stepped candidate digests by a fixed 2^128 digest units, which is one f128 ratio ulp only for boundaries in [0.5, 1); for small boundaries (a large-mean cdf(0) can sit at 2^-2000) the candidates landed many ulps away, diluting the straddle into ordinary sampling. Derive the step from the boundary's MantExp (2^(exp-128) in digest units, clamped to one digest unit when the ulp is finer than the digest grid), and assert the candidate window genuinely brackets the boundary whenever no candidate was clamped at the digest range's ends.

A separate single-platform job runs go run mutation_check.go on every push: a weakened test lets a mutant survive and a semantic change to the arithmetic kills a proven-equivalent mutant or orphans a target string, so the kill-rate claim stays continuously verified instead of decaying into a stale number. The mutant table is deliberately coupled to the exact arithmetic lines it guards; editing those lines requires updating the table. Also drop an out-of-repo doc path from the tool header.

Killed mutants fail the rapid tests by design, and each failure wrote a minimized reproducer under testdata/rapid that would then rerun as a stale regression case against the restored code. Pass -rapid.nofailfile in the kill suite.

The differential oracle shares the PMF recurrence with the implementation, so a shared algebra error would pass every differential test. selectExactRat computes the walk against the exact binomial CDF from first principles: stdlib big.Int.Binomial coefficients and pure integer cross-multiplication (t * T^money <= cdfNum(j) * (2^256-1)) -- no recurrence, no rounding, no floating point. TestSelectF128VsExactRat requires agreement within one boundary for money <= 50, over random inputs, near-maximum digests, and digests constructed to sit exactly on true CDF boundaries.

TestSelectF128Distribution mirrors TestSortitionBasic (which covers only the cgo path) for SelectF128 at toy and realistic stake scales with a fixed seed: summed selection weight must track N * money * p within 2%. Oracle-free and formula-free, so a gross semantic error fails it even if perfectly mirrored everywhere else. Both tests join the mutation campaign's kill suite via the TestSelectF128 pattern; the campaign still reports 33 mutants, 28 killed, 5 proven-equivalent, 0 unexpected.