Rename Mask Blur to Mask Spacer and narrow it to 2-6%

[marcinz606]

The contrast mask's radius control is renamed and its range tightened. Nothing about the operator changes.

Why the name

The slider is the sigma of the mask's blur, but the number reads backwards from a blur radius. A thin spacer makes the mask track the image more closely, so it cancels more of it: the print goes flatter at positive gamma and blooms at negative gamma. "Mask Blur" promised the print would get softer as the number went up, and the opposite happens. The spacer is the physical sheet that sets the unsharpness in the darkroom and it carries no such promise.

Why 2-6%

Measured per octave on a real Ektar scan, ratio against unmasked:

gamma +0.50 (reduction)      <2px   4    8    16   32   64  128  >128   global
  spacer 1%                  0.96  0.90 0.76 0.60 0.53 0.51 0.50 0.51   0.53x
  spacer 8%                  1.00  0.99 0.99 0.98 0.94 0.80 0.63 0.58   0.71x

gamma -0.50 (increase)
  spacer 1%                  1.04  1.11 1.27 1.42 1.47 1.49 1.50 1.50   1.48x
  spacer 8%                  1.00  1.01 1.01 1.03 1.09 1.24 1.40 1.44   1.37x

At 1% every band coarser than 8px collapses to a flat 0.5, which is a plain range scale plus a little of what Lab Sharpening already owns at 0.5-3.0px. The bottom of the old range was Grade under another label, so the floor moves to 2%. At 8% nothing under 64px moves at all, so the top moves to 6%.

The plane helper now clamps both ends rather than only the floor, so a value saved outside the new range cannot render wider than the slider can display.

Migration

mask_blur -> mask_spacer is one KEY_RENAMES entry. The field shipped in a merged PR but no release, so saved edits and sidecars can carry the old name.

Test

make all green: 4279 passed, 8 skipped. Includes a new test that a save under the old key still loads.