fix: select buffer home/end behaviour when textarea's content overflows

[danolekh]

fixes #521 and makes possible to properly implement #518

overview from clod:

Problem: Shift+cmd+arrow keys (select to buffer home/end) didn't work correctly in scrollable Textareas because selection anchor coordinates were captured relative to the wrong viewport position.

Root Cause: updateSelectionForMovement stored selection anchor using viewport-relative visual coordinates. When the viewport scrolled during a large cursor jump, the anchor became invalid.

Solution:

1. Store selection anchor using logical coordinates (row, col) which are document-absolute
2. Added scrollToCursor() to explicitly scroll viewport to cursor (bypassing the normal scroll-skip during selection)
3. After movement, convert anchor logical coords to visual coords based on current viewport

fix tested in alacritty & kitty terminals

preview
https://github.com/user-attachments/assets/7a61e7f2-6a87-4cbf-843f-b760d9619f76