modify() rejected only a non-finite radius or one above the maximum, then
clamped with r.max(0.0). A negative or zero radius reported success: Border
cleared the selection, Expand and Contract did nothing, and Feather 0 was
accepted although its documented range starts at 0.1.
Each command now checks the range in its params string (Border 1..200,
Smooth/Expand/Contract 1..500, Feather 0.1..1000). A radius outside it,
including NaN and infinity, returns BadParams naming that range, and the
selection is left as it was. The clamp is removed.