Counter.Add applied any delta, so a negative value would decrement a counter that must be monotonic, corrupting rate()-style downstream math. Ignore delta<0. Adds TestCounterRejectsNegativeAdd.
Mechanical fix applied by golangci-lint --fix. Pre-existing debt that fails the staticcheck QF1012 quickfix check; no behavior change. Clears the lint gate so subsequent commits in this package set pass.