v0.6.0: ScrollBox + TabBar — container components

ScrollBox:
- Container with vertical/horizontal scroll, viewport culling
- Scroll offset (:scroll-y, :scroll-x) with clamp to valid bounds
- Scrollbars rendered when content exceeds viewport
- Sticky scroll (auto-scroll to bottom on content change)
- Component protocol: component-children, component-layout-node

TabBar:
- Horizontal tab row with active/inactive styling
- tab-bar-next/prev (wraps around), tab-bar-select, tab-bar-handle-key
- Tab title rendering with overflow truncation (ellipsis)
- Component protocol: component-layout-node

26 scrollbox+tabbar tests, 100% GREEN:
171 total (27 backend + 58 box + 60 input + 26 scrollbox)

Review fixes applied:
- Removed duplicate definitions (org per-function blocks are prose-only)
- Fixed ASDF test path (../../tests/...)
- Version bumped to 0.6.0
- Added clamp-scroll export
- Added tab-bar-next/prev/select/handle-key tests
- Added scroll clamp boundary tests

src/components/container-package.lisp

@@ -0,0 +1,13 @@
+(defpackage :cl-tui.container
+  (:use :cl :cl-tui.backend :cl-tui.box :cl-tui.layout :cl-tui.input)
+  (:export
+   #:scroll-box #:make-scroll-box
+   #:scroll-box-scroll-y #:scroll-box-scroll-x
+   #:scroll-box-children #:scroll-by
+   #:sticky-scroll-p
+   #:clamp-scroll
+   #:tab-bar #:make-tab-bar
+   #:tab-bar-active #:tab-bar-tabs
+   #:tab-bar-add #:tab-bar-next #:tab-bar-prev
+   #:tab-bar-select #:tab-bar-handle-key
+   #:render))