Fix plane steps, custom-space restoration and gamut hue wrapping

[smelfungus]

Builds on #367.

Defects, each with a test that fails on dd4b25e:

• Plane steps build on the step not yet answered. Screen-reader actions taken before the caller's update recomposed the plane each started from the old position, so a step on one axis undid the one just taken on the other. Steps also start from the drawn position, so a NaN value no longer stays NaN whatever key is pressed.
• A controlled picker restores hue memory for a color in the app's own space. Its saver knew only the library's spaces and the picker's, so after recreation a grey came back at hue 0.
• RgbGamut.maxChroma and cusp wrap the hue first, as ColorValue does. Double.MAX_VALUE answered NaN, and a hue such as 1e20 was taken as a different angle.
• Equal RgbPrimaries hash alike when a coordinate is -0.0.

Docs:

• CSS Color 4 §14.2 allows three gamut-mapping algorithms; Css is the binary search with local MINDE, which holds lightness and hue to within a just-noticeable difference, and ChromaReduction holds them to rounding. README and the GamutMapping KDoc say which is which.
• The CMYK KDoc no longer calls the model reversible: many CMYK values give one RGB, and an RGB color whose highest channel is exactly 0 with another below it is singular.
• README: a requirements section, the saveable state in the Quick Start, and the classpath clash between 1.x and 2.x. It no longer names 1.2.1 as the latest release, which Maven Central does not have, and no longer says CI checks the gallery images, which are copies of the screenshot references.

Also clears three compiler warnings.

Module	JVM	Android host	wasm
color	242	229	229
color-compose	14	14	14
colorpicker-foundation	312	156	156
colorpicker-material3	173	4	4

apiCheck and the 30 screenshot baselines pass.