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>

CLAUDE.md

@@ -80,6 +80,10 @@ travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.
 
+An Int or Double param marked `.slider()` is shown in
+the interface as a slider with its minimum and maximum marked instead of a spin
+button; nothing else changes.
+
 A param can also be marked `.styled()`, which says a *named style* carries it
 rather than the stage it sits on: the font and the colour of a text stage, but
 never its words or its placement. The pool a style comes from is named by