Round 2 of consolidating CSS Variables

Problem/Motivation

In #3599680: Consolidate, merge, and refactor Gin's CSS variable's into Admin theme's original variables., we took the variables from Gin and Claro and consolidated them into one architecture with a new namespace. We were in a bit of a hurry to get this into 11.4.0, so there's some more tidying up that needs to be done.

Steps to reproduce

There are multiple legacy variables that are not in use, and multiple that have only one usage. These should be refactored out if possible.

We'll also likely do a bit of general cleanup.

Audit

Usage counts for every variable defined in
core/themes/default_admin/css/_variables/variables-color.pcss.css,
measured across all *.pcss.css files in the theme.

🔴 Zero usage — fully dead (26)

Not referenced anywhere. First-pass deletion candidates.

The entire red ramp is dead because the status tokens
(--admin-color-error, --admin-color-text-error, etc.) use
literal hex values, not the ramp.

🟠 Used only by other tokens (OUTSIDE = 0, TOTAL ≥ 1) — inline candidates

These primitives feed only semantic tokens inside _variables/. Each could be
inlined into its consumer and deleted.

--admin-shadow-color-1…4 only feed the --admin-shadow-1/2
composites — that's by design, probably keep.

🟡 Low direct usage (OUTSIDE 1–2) — worth a look

Testing instructions

There should be no changes to the UI. The CSS is being moved around and refactored.

Test the site at various breakpoints, various browsers, and in dark and light mode. Use https://github.com/mherchel/ddev-drupal-admin-vrt to do automated visual regression tests