feat(vxfw): add ListView jumpToItem

[rockorager]

Summary

• add ListView.jumpToItem() for direct cursor jumps without walking intermediate children
• add ListView.scrollToBottom() for direct bottom scrolling when the item count is known
• clamp jumps when item_count is known
• get known counts from slice-backed ListViews
• add tests for large builder jumps, bottom scrolling, and clamping

Test

• zig build test

Fixes #334