fix(vtable): keep headers after setRecords at scroll edge

[fangsmile]

Summary

• Preserve and restore scroll positions when ListTable.setRecords rebuilds the scenegraph.
• Add a local debug demo for issue [Bug] 列很多时，有左右固定列， 横向滚动条滚动到最右边， 此时setRecords, 表头出现空白 #5278 with left/right frozen columns and many columns.
• Add a regression test covering both the scrolled header and left frozen header after setRecords([]).

Test plan

• Verified in local examples page: clicking scroll right + setRecords([]) results in records=0 and both Order ID and Sales294...Sales299 headers remain visible.
• Ran commit hooks: rush lint-staged and rush commitlint passed.
• npm run compile is blocked by existing workspace resolution errors for @visactor/vtable-editors.
• Pre-push rush test --only tag:package is blocked by unrelated package failures; branch was pushed with --no-verify after maintainer confirmation.

🤖 Generated with Claude Code