fix: selection follows scroll and auto-scrolls at viewport edges

[190km]

Summary

• Scroll sync during active selection: When scrolling via mouse wheel while dragging a text selection, the selection coordinates now stay in sync with the changing display offset. Previously the selection would drift and highlight wrong content.
• Auto-scroll at viewport edges: When dragging a selection above or below the terminal body, the terminal now auto-scrolls in that direction and extends the selection into the newly visible content. Scroll speed scales with pointer distance from the edge.
• Helper method: Added auto_scroll_selection() to atomically scroll the terminal and update selection coordinates, preventing any mismatch between scroll state and selection state.

Test plan

• Start a selection drag in a terminal with scrollback history
• While still dragging, scroll with the mouse wheel — verify the selection highlight stays on the correct text
• Drag the selection above the terminal top edge — verify the terminal scrolls up into history and the selection extends
• Drag the selection below the terminal bottom edge — verify the terminal scrolls down toward recent output and the selection extends
• Release the mouse and copy — verify the copied text matches the highlighted selection