Feat v0.6.11 CSS deduplication (#46)

Retire old primitives.css button/toast/popup-menu/dropdown/tabs systems.
sw-primitives.css is now the single source of truth.

- Migrate 29 files from .btn-* to .sw-btn .sw-btn--{variant} .sw-btn--{size}
- Delete old toast, popup-menu, dropdown, tabs CSS (no JS references)
- Resolve .sw-dropdown and .sw-tabs class name collisions
- Resolve .settings-section duplicate (modals.css removed, surfaces.css authoritative)
- Add .sw-btn--success variant, --bg-active CSS variable
- Fix sdk-test-runner wrong variable names, icd-test-runner button fallbacks
- Add scripts/audit-css-collisions.sh

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

ROADMAP-UI.md

@@ -51,9 +51,9 @@ Shipped. Single layout model: `body → shell → surface`. CSS `zoom` replaces
 `transform: scale()`. `100dvh` fallbacks everywhere. Extension surfaces use
 `100%` instead of `100vh`. Dead shell layout code deprecated. See CHANGELOG.md.
 
-### v0.6.11 — CSS Deduplication
+### v0.6.11 — CSS Deduplication ✅
 
-Kill the old primitive system. One class per concept.
+Shipped. Old primitive system retired. One class per concept. See CHANGELOG.md.
 
 | Step | Description |
 |------|-------------|
@@ -118,7 +118,7 @@ v0.6.9  Session Lifetime Config ✅ SHIPPED
   ↓
 v0.6.10 Viewport Foundation     ✅ SHIPPED
   ↓
-v0.6.11 CSS Deduplication       ← Can't audit styling until there's one source of truth
+v0.6.11 CSS Deduplication       ✅ SHIPPED
   ↓
 v0.6.12 Extension CSS Isolation ← Scoping requires the kernel CSS to be stable first
   ↓