feat: Improve selection handling for buffer trims and scrolling

[IvanJosipovic]

Summary

Update selection handling in XTerm.NET to better match xterm.js.

This changes selection from a viewport-relative model to a buffer-anchored model so selection stays attached to the underlying buffer content when the viewport scrolls.

What changed

• Store selection rows in absolute buffer coordinates inside SelectionManager
• Translate viewport-relative row input to absolute buffer rows in:
  • StartSelection
  • UpdateSelection
  • IsCellSelected
• Update GetSelectionText() to read directly from absolute buffer lines instead of buffer.YDisp + row
• Update SelectAll() to cover the full buffer, including scrollback, not just the visible viewport
• Add trim handling so selection adjusts or clears when old scrollback lines are recycled
• Add TerminalBuffer.Trimmed so selection can respond when buffer lines are dropped

Why

The previous selection implementation was tied to viewport rows:

• scrolling changed which buffer lines those stored rows referred to
• selected text could appear to move with the viewport
• selection text and rendering did not remain stably anchored to the original buffer content

xterm.js behaves differently. Its selection model is buffer-oriented and adjusts selection when scrollback is trimmed. This change moves XTerm.NET closer to that behavior.

Tests

Added focused tests covering:

• selected text remains anchored when the viewport scrolls
• IsCellSelected follows the selected buffer line across viewport movement
• SelectAll includes scrollback, not just the visible rows
• selection clears correctly when trim removes the selected lines

Validation:

• dotnet test src/XTerm.NET.Tests/XTerm.NET.Tests.csproj
• passed 584/584

[tomlm]

Thanks!

[tomlm]

I pushed a new package