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/tabbar.lisp

@@ -0,0 +1,51 @@
+(in-package #:cl-tui.container)
+
+(defclass tab-bar (dirty-mixin)
+  ((tabs :initform nil :initarg :tabs :accessor tab-bar-tabs :type list)
+   (active :initform nil :initarg :active :accessor tab-bar-active)
+   (layout-node :initform (make-layout-node) :accessor tab-bar-layout-node)
+   (focusable :initform t :accessor tab-bar-focusable)))
+
+(defun make-tab-bar (&key tabs active)
+  (make-instance 'tab-bar :tabs (or tabs nil) :active active))
+
+(defun tab-bar-add (tb id title)
+  (setf (tab-bar-tabs tb) (nconc (tab-bar-tabs tb) (list (list :id id :title title))))
+  (unless (tab-bar-active tb) (setf (tab-bar-active tb) id)) id)
+
+(defmethod component-layout-node ((tb tab-bar)) (tab-bar-layout-node tb))
+
+(defun tab-bar-next (tb)
+  (let* ((tabs (tab-bar-tabs tb)) (current (tab-bar-active tb))
+         (ids (mapcar (lambda (tab) (getf tab :id)) tabs))
+         (pos (position current ids)))
+    (when pos (let ((next (nth (mod (1+ pos) (length ids)) ids)))
+                (setf (tab-bar-active tb) next) (mark-dirty tb)))))
+
+(defun tab-bar-prev (tb)
+  (let* ((tabs (tab-bar-tabs tb)) (current (tab-bar-active tb))
+         (ids (mapcar (lambda (tab) (getf tab :id)) tabs))
+         (pos (position current ids)))
+    (when pos (let ((prev (nth (mod (1- pos) (length ids)) ids)))
+                (setf (tab-bar-active tb) prev) (mark-dirty tb)))))
+
+(defun tab-bar-select (tb id) (setf (tab-bar-active tb) id) (mark-dirty tb))
+
+(defun tab-bar-handle-key (tb event)
+  (case (key-event-key event) (:left (tab-bar-prev tb) t) (:right (tab-bar-next tb) t) (t nil)))
+
+(defmethod render ((tb tab-bar) backend)
+  (let* ((ln (tab-bar-layout-node tb)) (y 0)
+         (w (if ln (layout-node-width ln) 80))
+         (active-id (tab-bar-active tb)) (tabs (tab-bar-tabs tb)) (x-pos 0))
+    (dolist (tab tabs)
+      (let* ((id (getf tab :id)) (title (getf tab :title))
+             (label (format nil " ~A " title)) (label-len (length label))
+             (is-active (eql id active-id))
+             (fg (if is-active :accent :text-muted))
+             (bg (if is-active :background-element nil)))
+        (when (>= (+ x-pos label-len 2) w)
+          (draw-text backend x-pos y "..." :text-muted nil) (return))
+        (draw-text backend x-pos y label fg bg)
+        (incf x-pos (+ label-len 2)))))
+  (values))