fix: prevent resize observer feedback loop in vertical mode

[tomkp]

Summary

• Rounds container size to nearest integer to prevent sub-pixel variations from triggering updates
• Uses a ref to track last observed size and skip redundant updates
• Prevents infinite re-render loops in vertical mode with certain CSS layouts

Root Cause

When using direction="vertical" with certain container sizing setups (like CSS solution #3 from the docs), the ResizeObserver would fire repeatedly with tiny sub-pixel variations (768.0, 768.1, 768.2, etc.). Each update would trigger a state change, which affected layout, which triggered another ResizeObserver callback, creating an infinite loop.

Solution

Round the observed size to the nearest integer before comparison and state updates. This prevents sub-pixel "noise" from causing updates while still responding to meaningful 1+ pixel size changes.

Test Plan

• Added test verifying sub-pixel variations don't cause state updates
• Added test verifying significant size changes still work correctly
• All 107 tests pass
• Linting, formatting, and type checking pass

Fixes #873