fix: add pixel overscan

[ImLunaHey]

What changed

• add an overscanPixels option alongside item-count overscan
• combine pixel and item buffers using the existing prefix-size tree
• keep the zero-pixel hot path free of extra tree searches
• document the high-velocity scrolling use case

Why

Native composited scrolling can move farther than a small item-count buffer before a framework commits its next render. With variable-height rows, an item count also does not represent a predictable physical buffer.

Validation

• full package check
• 20 tests with 100% statement, branch, function, and line coverage
• package audit passes
• smoke benchmark retains roughly 2.25 million visible-range updates per second