refactor(tokens): make token layer authoritative and wire to layout

- Imported tokens.css directly into BaseLayout.astro.
- Removed legacy :root variable definitions from chapters.css, skills-review/styles.css, rules/styles.css, and styles.css.
- Added self-hosted --font-sans and --font-mono to tokens.css and updated legacy font stacks.
- Removed base.css.
- Added a build-output check in check-tokens.mjs to ensure the token layer is loaded in dist html files.

src/layouts/BaseLayout.astro

@@ -10,6 +10,8 @@ const { title, description, lang = 'en' } = Astro.props;
 // Self-hosted Manrope and DM Mono. Lives in `public/fonts/` rather than the
 // bundled CSS because the legacy stylesheets @import the same file, so there
 // is exactly one copy of the faces. See public/fonts/fonts.css.
+import tokensStylesheet from '../styles/tokens.css?url';
+
 const fonts = `${import.meta.env.BASE_URL}fonts/fonts.css`.replace('//', '/');
 ---
 
@@ -20,6 +22,7 @@ const fonts = `${import.meta.env.BASE_URL}fonts/fonts.css`.replace('//', '/');
     <meta name="viewport" content="width=device-width, initial-scale=1" />
     <title>{title}</title>
     <meta name="description" content={description} />
+    <link rel="stylesheet" href={tokensStylesheet} />
     <link rel="stylesheet" href={fonts} />
     <slot name="styles" />
   </head>