v0.2.0: Dirty tracking — dirty-mixin, mark-clean, mark-dirty

- dirty-mixin class with dirty slot (initform t)
- mark-clean clears dirty flag
- mark-dirty sets dirty flag
- 3 tests: default-dirty, clean, dirty-cycle
- ROADMAP.org: v0.2.0 all tasks DONE
- 31 component tests, 100% GREEN

src/components/dirty.lisp

@@ -0,0 +1,14 @@
+(in-package :cl-tui.box)
+
+;; ── Dirty Tracking ─────────────────────────────────────────────
+
+(defclass dirty-mixin ()
+  ((dirty :initform t :accessor dirty-p)))
+
+(defgeneric mark-clean (component)
+  (:method ((c dirty-mixin))
+    (setf (dirty-p c) nil)))
+
+(defgeneric mark-dirty (component)
+  (:method ((c dirty-mixin))
+    (setf (dirty-p c) t)))