AppHeader is rendered at the top of the terminal by RenderLoop, similar to
StatusBar at the bottom. Views declare header content via .appHeader { }
ViewBuilder modifier. Supports standard (thin divider) and block (half-block
with appHeaderBackground) appearance. Hidden when no content is set.
Diff cache invalidates on header height changes to prevent ghosting.
Update IDETemplateMacros.plist and replace headers in 136 Swift files with
new format: 🖥️ TUIKit — Terminal UI Kit for Swift. Remove sdsd.swift template draft.