Primitives and Blocks                                         
                                                              
painted is built from a small set of immutable render-layer   
value types. These are the inputs to every higher-level       
feature — composition, buffers, the TUI, widgets.             
                                                              
Primitives  Style, Cell, Span, Line                           
Rectangles  Block                                             
                                                              
Style                                                         
  Style is an immutable bundle of attributes — colors plus    
  bold/italic/underline/reverse/dim. Styles combine via       
  merge(), where the overlay wins.                            
                                                              
  base = Style(fg='blue', bold=True)                          
  merged = base.merge(Style(italic=True))                     
                                                              
  bold                                                        
  red                                                         
  green                                                       
  blue                                                        
  dim italic                                                  
  reverse cyan                                                
  Style attributes, rendered live                             
                                                              
Cell                                                          
  Cell is the atom: one character plus one Style. Most code   
  manipulates Blocks rather than individual cells.            
                                                              
Span and Line                                                 
  Span is text plus Style, measured in display columns        
  (wide-char aware). A Line is a tuple of spans that paints   
  into a buffer or converts to a Block.