refactor: CSS tasks 7-9 — split components into landmarks, forms, navigation

Extract from monolithic 03-components.css:
- 03-landmarks.css: nav, article, section, hgroup, dt/dd, footer
- 04-forms.css: buttons, inputs, selects, checkboxes, validation
- 06-navigation.css: sidebar nav, TOC aside

03-components.css retains: accordion, dialog, breadcrumb, steps.
All files will be renumbered to final scheme in next task.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.css

@@ -14,7 +14,10 @@
 @import "./layers/00-reset.css";
 @import "./layers/01-tokens.css";
 @import "./layers/02-semantic.css";
+@import "./layers/03-landmarks.css";
+@import "./layers/04-forms.css";
 @import "./layers/03-components.css";
+@import "./layers/06-navigation.css";
 @import "./layers/04-data-attrs.css";
 @import "./layers/05-utilities.css";
 @import "./layers/06-charts.css";