feat: EZA_COLORS options to tweak the range and style of color scale for age/size

An idea for configurability of color scale range & colors:

Have `sN` to configure the gradient and the range for the color scale system, where `N` is a number starting at 1.
The number of options with incrementing number determines the number of level of the color gradient, and the values determines the color for each level.

So for example with color tweaks for `s1`, `s2`, `s3`, `s4` would make a gradient with 4 colors.
Similarly, with color tweaks for `s1`, `s2` would make a gradient with only 2 colors.

A slight alternative could be to separate the size/age config: `aN` configures the scale for `age`, and `sN` configures the scale for `size`.

No files, tests, or entry points are named. Start by locating the existing EZA_COLORS parsing and the age/size color-scale implementation; clarify whether sN should configure both scales or whether aN and sN should be separate, then verify that numbered options produce the requested gradient levels and colors.

Written by the indexing model from the issue text.