feat: add OpenSpec specs and changes for ASW restructure

Specs: repo-structure, 10 framework layer specs, packs, site.
Changes: repo-restructure (10 tasks), css-refactor (12 tasks),
legacy-import (proposal + triage categories).

Supersede docs/css-refactor-plan.md in favor of OpenSpec change.

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

openspec/specs/framework/navigation/spec.md

@@ -0,0 +1,27 @@
+# Spec: Navigation (Layer 06)
+
+File: `src/layers/06-navigation.css`
+
+## Purpose
+
+Navigation patterns that go beyond the basic `nav` landmark. Sidebar navigation, table of contents, dropdown menus, navigation state management.
+
+## Scope
+
+### In scope
+- `nav[data-nav="sidebar"]` — sidebar navigation tree
+- `aside[data-toc]` — in-page table of contents
+- Dropdown menus within navigation
+- Active/current page indicators (`aria-current`)
+- Mobile navigation patterns
+- Navigation collapse/expand behavior
+
+### Out of scope
+- The `body > nav` landmark itself (→ landmarks layer)
+- Breadcrumbs (→ components layer)
+
+## Constraints
+
+- References only semantic aliases from `01-tokens.css`
+- Navigation must be keyboard-accessible
+- Active states use `aria-current`, not classes