Module render 

Turning a line of text plus the selections covering it into styled spans.

Split out of lib.rs because this is where display-column arithmetic lives, and it is the part most likely to grow: highlighting arrives in M2.5 and has to compose with selection styling rather than fight it.

Functions

styled_line

Build one rendered line, splitting it into spans wherever the selection state changes.

window

Drop left_col display columns from the front of a line.