Make Tile swap slow at a low Swap rate, on a curved slider

A tile has to come back to the cut it started with for the clip to
loop, so every tile swapped at least twice a clip, and below that
rounding left every tile still: the bottom of the slider was either
frozen or busy. Swap rate is now the average per tile: below two swaps
a clip only a share of the tiles swap (twice each), fewer the lower it
goes, and above it each tile rounds up or down by its own draw.

ParamSpec::slider() takes an optional curve: the scale runs over the
slider's travel and sliderValue/sliderPosition map it onto the number
as position^curve. Swap rate uses a curve of 3, so the low end gets
most of the slider; the command line and project file are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -10,6 +10,12 @@ commit that made it. The interface shows this file under Changelog in its menu.
 - The Swap rate of Tile swap is a slider marked with its range, and goes
   down in steps of 0.01 rather than 0.1, so a clip can swap its tiles much
   more slowly. Its top end is now 5 swaps a second (it was 30).
+- At a low Swap rate, Tile swap now moves only some of its tiles, fewer the
+  lower the rate, instead of swapping every tile at least twice a clip (or,
+  below that, none at all). Each step down the bottom of the slider now
+  looks slower than the one above it.
+- The Swap rate slider is curved: its low end takes up most of its length
+  (1 swap a second sits just past the middle), so slow rates are easy to set.
 
 ## [0.8.1] - 2026-09-26