The degenerate-magnitude shortcut in ProjectToSimplex::repair scans
`decision` for the argmax and concentrates all mass there. cargo
mutants found that the strict-greater scan was unpinned: replacing
`>` with `>=` (which would shift the argmax to the last tied
index) and `>` with `==` (which would silently skip larger
values further along) both survived.
Two tests:
- A 3-element vector with two tied maxima at the front pins that the
scan keeps the first index on a tie.
- A 3-element vector whose argmax is at index 1 pins that the scan
actually walks past the start when later values are larger.
Other mutants in this file (the `>` ↔ `>=` threshold check at line
106, the `*` ↔ `+` in the threshold constant, the `-` ↔ `+` /
`/` in the tau-fallback initializer, and the `>` ↔ `>=` in the
projection loop's rho update) are equivalent mutants for non-pathological
inputs: the normal-path and shortcut-path math converge to the same
projection result for any input the operator is documented to handle.
Leaving them in the residue.
swaits
47cb1d5f79