Add read-only editor mode

[tig]

Summary

• add Editor.ReadOnly and guard editor edit commands, typed input, tab indentation, replacement APIs, and undo/redo
• keep navigation and selection commands active while read-only
• add a ted read-only constructor/CLI flag path and guard ted paste/cut/undo/redo menu actions
• mark read-only complete in the feature spec and roadmap

Validation

• dotnet build Terminal.Gui.Text.slnx - passed
• dotnet run --project tests/Terminal.Gui.Text.Tests - 212 passed
• dotnet run --project tests/Terminal.Gui.Editor.Tests - 78 passed
• dotnet run --project tests/Terminal.Gui.Editor.IntegrationTests - 105 passed
• dotnet format Terminal.Gui.Text.slnx --exclude third_party/ - passed
• dotnet format Terminal.Gui.Text.slnx --verify-no-changes --exclude third_party/ - passed
• dotnet jb cleanupcode Terminal.Gui.Text.slnx --profile="Full Cleanup" - blocked because the profile is not defined
• dotnet jb cleanupcode Terminal.Gui.Text.slnx - reported no items to cleanup (tool exited 3)