fix: preserve scroll position in inverse mode

[ayushdwivedi-stack]

Fixes #418

Summary

When new items are prepended in inverse mode, the scroll position was not compensated, causing the visible content to jump.

This change preserves the user's scroll position by:

• Saving scrollHeight and scrollTop before loading more items.
• Calculating the scroll height difference after new items are rendered.
• Adjusting scrollTop by that difference.

Tests

• Added a test for preserving scroll position after new items are loaded in inverse mode.
• Verified existing inverse mode behavior.

Validation

• 68 tests passed
• TypeScript check passed