Immediate-Mode-Rendering

The rat starts painting. We implement the rendering layer: immediate-mode drawing, a three-zone layout with Layout::vertical, a task table with StatefulWidget and row highlighting, a context-sensitive command bar that changes with InputMode, and the subtle Block gotcha that cost a few minutes of debugging.

We close the series by exploring what it means to migrate from CLI to TUI with ratatui: how the interaction model changes, what frictions Rust introduces with ownership and &mut in a persistent event loop, and why hexagonal architecture absorbs the change without surgery.