[codex] Implement D1 tab handling

[tig]

Summary

Refs #37 and #43.

This chooses option (b) from the experiment prompt: implement the VisualLineBuilder slice needed for D1, then implement tab handling on top of it. This is not the full B1/B2 pipeline; cache invalidation, folding markers, and line-number background migration remain out of scope and are called out in the final run report.

Changes:

• add grapheme-aware visual-line elements, including TabElement
• rename Editor.TabWidth to IndentationSize and add ConvertTabsToSpaces / ShowTabs
• route editor drawing and tab hit-testing through visual-line mapping
• add Tab, Shift+Tab, block indent/unindent, and indentation-aware Backspace
• update ted status/options UI for indentation size, visible tabs, and spaces-on-tab
• add public API notes and the required Codex final report

Validation

• dotnet build Terminal.Gui.Text.slnx
• dotnet run --project tests/Terminal.Gui.Text.Tests
• dotnet run --project tests/Terminal.Gui.Editor.Tests
• dotnet run --project tests/Terminal.Gui.Editor.IntegrationTests
• dotnet format Terminal.Gui.Text.slnx --verify-no-changes --exclude third_party/

Note: dotnet tool restore succeeded. dotnet jb cleanupcode Terminal.Gui.Text.slnx --profile="Full Cleanup" failed because the JetBrains CLI did not recognize the solution cleanup profile in this .slnx checkout; cleanup against the touched projects ran with the built-in reformat profile.

[tig]

CI note: macOS and Windows passed on run 25613380302. Ubuntu fails in the ReSharper cleanupcode step before tests with No items were found to cleanup from dotnet jb cleanupcode Terminal.Gui.Text.slnx --profile="Built-in: Full Cleanup" --no-build --exclude="third_party/**/*".

I checked current develop run 25613103283 and it fails the same way, so I’m treating this as an existing workflow/tooling issue. Per the task constraints I did not edit .github/workflows; the same rationale is recorded in specs/runs/test-codex-final.md.