Show Tile swap's Swap rate as a slider, and let it go slower

A new ParamSpec::slider() marks an Int or Double param to be shown in
the interface as a Gtk::Scale with its minimum and maximum marked under
the ends, rather than a spin button; the command line and the project
file are unchanged. Swap rate uses it, and now runs from 0 to 5 in
steps of 0.01 (was 0 to 30 in steps of 0.1): with every tile swapping
on its own, even 0.1 swapped the canvas many times a second.

Version 0.8.2.

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

CHANGELOG.md

@@ -4,6 +4,13 @@ Every release of Effector, newest first. The version lives in one place, the
 `project()` line of `CMakeLists.txt`, and each release is a `vX.Y.Z` tag on the
 commit that made it. The interface shows this file under Changelog in its menu.
 
+## [0.8.2] - 2026-09-30
+
+### Changed
+- 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).
+
 ## [0.8.1] - 2026-09-26
 
 ### Added