Add column selection for vertical multi-caret

[Copilot]

Vertical multi-caret supported carets-only placement, but column/box selections were not created or rendered. This adds VS Code-style column selection behavior while preserving the terminal-specific Alt+drag modifier.

• Column selection input

  • Alt+drag with horizontal extent creates one selection per row.
  • Zero-width Alt+drag still creates bare vertical carets.
  • Reversed drags and short-line clamping are handled without inserting padding.
  • Ctrl+Shift+Alt+Arrow/PgUp/PgDn creates/extends keyboard column selections.

• Rendering

  • Additional-caret selection spans now render with the active selection role.
  • Existing additional caret overlay rendering remains unchanged.

• Indent/unindent behavior

  • Multi-caret Tab / Shift+Tab now preserves primary and per-caret selections across block indentation.
  • Column selections remain usable after indent/unindent.

• Docs/specs

  • Updated multi-caret help and specs with current column-selection behavior.
  • Documented D1–D5 parity/deviation notes, including keyboard column-select as not a deviation.

Example behavior:

Alt+drag from column 1 to column 3:

abcd
abcd
abcd

typing "x" replaces each selected row range:

axd
axd
axd

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• www.jetbrains.com
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/.nuget/packages/jetbrains.resharper.globaltools/2026.1.1/tools/net8.0/any/cleanupcode.unix.runtimeconfig.json /home/REDACTED/.nuget/packages/jetbrains.resharper.globaltools/2026.1.1/tools/net8.0/any/cleanupcode.exe Terminal.Gui.Editor.slnx (dns block)
  • Triggering command: /usr/share/dotnet/dotnet /usr/share/dotnet/dotnet exec --runtimeconfig /home/REDACTED/.nuget/packages/jetbrains.resharper.globaltools/2026.1.1/tools/net8.0/any/cleanupcode.unix.runtimeconfig.json /home/REDACTED/.nuget/packages/jetbrains.resharper.globaltools/2026.1.1/tools/net8.0/any/cleanupcode.exe Terminal.Gui.Editor.slnx --profile=TG.Editor Full Cleanup (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)