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/landmarks/spec.md

@@ -0,0 +1,29 @@
+# Spec: Landmarks (Layer 03)
+
+File: `src/layers/03-landmarks.css`
+
+## Purpose
+
+Styling for HTML landmark elements — the structural bones of a page. These elements convey document structure to both browsers and screen readers.
+
+## Scope
+
+### In scope
+- `body > nav` — global navigation bar
+- `article` — content card or post
+- `aside` — sidebar panels
+- `section` — content grouping
+- `body > footer` — global footer
+- `hgroup` — heading groups
+- `header` within landmarks
+- `dt`, `dd` — definition lists (structural, not component)
+
+### Out of scope
+- Navigation menus and dropdowns (→ navigation layer)
+- Sidebar nav behavior `nav[data-nav="sidebar"]` (→ navigation layer)
+- Article styling within specific layouts (→ layouts layer)
+
+## Constraints
+
+- References only semantic aliases from `01-tokens.css`
+- Landmark styling should be layout-independent — works in any `data-layout`