feat(themes): Built-in Solarized and One light/dark themes (#85)

Adds Solarized Light, Solarized Dark, One Light and One Dark as built-in themes across the workbench, editor, terminal and Mermaid, keeping the original palettes (MIT notices in THIRD_PARTY_LICENSES.md, mapping in docs/themes.md).

- Native window colours are read from tokens.css at build time (`theme_frame.rs`) instead of a second colour table.
- Startup restores the built-in palette before first paint.
- Editor schemes, the terminal and Mermaid refresh on switches between themes of the same kind.
- The plugin packages of the same themes are removed.
- A test requires every built-in palette theme to define each token (only listed ones may inherit from the base theme), so new tokens can't silently miss them.

Co-authored-by: Claude <[email protected]>
Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/editor/core/colorSchemeRuntime.ts

@@ -28,7 +28,7 @@ export function installColorScheme(): () => void {
   const observer = typeof MutationObserver !== "undefined" ? new MutationObserver(apply) : null;
   observer?.observe(document.documentElement, {
     attributes: true,
-    attributeFilter: ["data-theme", "data-contrast", "data-plugin-theme"],
+    attributeFilter: ["data-theme", "data-contrast", "data-color-theme", "data-plugin-theme"],
   });
   return () => {
     unsubscribe();