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fix: correct border width calculation in renderHorizontalEdge

The renderHorizontalEdge function had an off-by-one error that caused
borders to be rendered with incorrect width. This fix:

1. Properly calculates the middle section width by subtracting both
   left and right widths from the total width
2. Fixes the loop logic to fill exactly the required middle width
3. Adds comprehensive tests to verify correct border rendering

This resolves issues where borders would overflow or underflow their
intended width, particularly noticeable with wider borders.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>