#11684 Fix tab scroll range fitting complexity

Summary

• add behavior coverage for preserving offsets and fitting active tabs at scale
• replace repeated range scans with one-pass accumulated width subtraction

The renderer keeps Ratatui's standard model: compute bounded Rects and write widgets into the frame buffer. See https://docs.rs/ratatui/latest/ratatui/struct.Frame.html and https://docs.rs/ratatui/latest/ratatui/layout/struct.Rect.html.

Verification

• rustfmt
• git diff --check
• focused cmux-tui hosted workflow: https://github.com/manaflow-ai/cmux/actions/runs/33639066113

Two commits are intentional: the first adds the failing behavior tests, the second adds the fix.

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Summary by cubic

Fixes tab scroll range fitting so large tab sets no longer cause quadratic work. Replaces repeated range scans with a one-pass accumulated width subtraction and adds tests for preserving requested offsets and fitting active tabs at scale.