fix: avoid selection gap with frozen rows

[fangsmile]

Summary

• fix visible body row range calculation when frozenRowCount is enabled
• avoid selection border gap while dragging the vertical scrollbar with a selected column
• add an issue-5027 debug demo and a regression test

Reproduction

• open Debug / issue-5027 in the examples site
• the demo preselects the age column with frozenRowCount: 5
• drag the vertical scrollbar and observe the selected column border

Root Cause

• getBodyVisibleCellRange() and getBodyVisibleRowRange() may derive the body visible start row incorrectly when top frozen rows exist
• the selection border body segment then uses the wrong visible row range and appears disconnected from the frozen area

Verification

• reproduced in browser with the new debug demo
• verified the selected column border stays continuous after scrolling
• added regression coverage for the frozen-row visible range API