feat(D1): implement tab handling per issue #37

[tig]

Summary

• Rename Editor.TabWidth → IndentationSize — mirrors AvaloniaEdit's TextEditorOptions.IndentationSize (R3 compliant)
• Add ConvertTabsToSpaces (bool, default false) — governs Tab key behavior, not document content
• Add ShowTabs (bool, default false) — renders → glyph at tab positions
• Add Tab / Shift+Tab key handlers — insert, block indent/unindent with single undo step (R5)
• Mouse snap to nearest edge — click inside tab span snaps to nearest logical edge (§7)
• Updated ted demo — Indent Size control, Convert Tabs To Spaces menu toggle, ShowTabs status checkbox
• 21 new tests across Editor.Tests and IntegrationTests; all 337 tests pass

B1 dependency — choice (c): stopgap, explicitly owned

The spec (§8 D1) says D1 depends on B1 (VisualLineBuilder pipeline) and without B1 "becomes another welded shortcut and is rejected." I chose to ship a working implementation and own the R1/R2 violation:

What violates R1/R2:

• Tab rendering in DrawLineContent still operates char-by-char inside OnDrawingContent
• No TabElement in a visual-line pipeline (B1 doesn't exist yet)

What's clean:

• All interim helpers marked // TODO(VisualLineBuilder) for deletion when B1 lands
• Tab key behavior is self-contained in Editor.Commands.cs
• Properties mirror AvaloniaEdit names (R3)
• Block operations use RunUpdate() for single undo step (R5)
• Mouse nearest-edge snap is correct

Skipped

• Backspace-at-indentation (§5) — marked as stretch goal in the spec
• Full grapheme-aware rendering rewrite (§6 interim) — the right fix is B1, not a second interim hack
• IIndentationStrategy plumbing — separate work item D7

Test plan

• dotnet run --project tests/Terminal.Gui.Text.Tests — 212 passed
• dotnet run --project tests/Terminal.Gui.Editor.Tests — 44 passed
• dotnet run --project tests/Terminal.Gui.Editor.IntegrationTests — 81 passed
• dotnet format --verify-no-changes — clean
• dotnet jb cleanupcode — no changes

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