Module movement 

Cursor motion over a rope. Pure functions on (text, position), so every rule here is testable without constructing a crate::Buffer.

Char-wise, not grapheme-wise, for now. ADR-0006 §1 puts grapheme awareness at this layer — one arrow press should cross a whole cluster, so an emoji with a skin-tone modifier takes one keypress and not three. That refinement belongs here when it lands and changes nothing above; the change representation stays char-indexed either way, which is exactly why the ADR separated the two.

Functions

column_of

The column of pos within its line, in chars.

down

left

line_end

Last char of the line pos is on, before its terminator — pressing End should land at the end of the visible text, not on the far side of the newline.

line_of

The line pos sits on.

line_start

First char of the line pos is on.

right

up

Move one line up, aiming for goal if the caller is tracking a sticky column.