[Plan] Maintainer reviews before an alpha

Problem/Motivation

Design System API proposes a shared design contract for Drupal: semantic design tokens with defaults, which themes fill (directly or through adapters) and modules build against, so contrib UI looks right under any admin or frontend theme. The concept is written up, and a v0 runs on Drupal 12.0.0-beta1: a token vocabulary on core's Design Tokens API, a resolver with per-context values and provenance, adapters for Default Admin and Claro, the Media Library restyled only through the contract, a CSS linter, and a YAML editor for site authors.

Before an alpha we need the people who know each area to check the direction. Each child issue has a short review brief with 3–5 specific questions, what was built, screenshots and how to try it in the demo site.

Proposed resolution

One review issue per area. Suggested reviewers are the maintainers listed for that area in core's MAINTAINERS.txt; nobody has committed to anything. If you'd rather not be listed, or know someone better placed, please say so here.

Related core work: #3531854: Add a Design Tokens & CSS variables API (Design Tokens API), #3622909: Add remaining design tokens value types (remaining DTCG value types: color, border, shadow, …), #3590100: Add support for design tokens aliases (token references), #3624211: Add SDC specific design tokens (SDC component tokens).

Remaining tasks

• Reviews in the child issues; answers summarized under "What we heard" in each brief.
• Decisions from the reviews folded into the concept.
• Core value types in #3622909: Add remaining design tokens value types (color, border, shadow first); then drop the module's interim color and shadow plugins.
• Decide where adapters for contrib themes live (Claro is contrib on Drupal 12; Gin once it supports Drupal 12).
• Alpha release once the vocabulary and the review outcomes are settled.

User interface changes

None in core. The module adds Appearance » Design system (author values per theme).

API changes

None in core yet. Candidates for core come out of the reviews (value types, references, $extensions).