Optimize TextRangeCollection.getItemContaining

[Zaczero]

This change improved the performance on openstreetmap-ng codebase by about 1%.

14.6s → 14.4s (-1.1% ± 0.4%)

The generic getIndexContaining supports sparse arrays ((T | undefined)[]) and does extra defensive work. In our hot paths (token/line collections), the array is dense and ordered, so this version:

• avoids undefined checks and extra branching
• uses a tight binary search loop
• adds a cheap “gap” early-out to avoid extra iterations when the position is clearly between adjacent ranges