fix(vtable): enlarge fill handle hit area

[fangsmile]

Summary

• enlarge the fill handle pointer hit target from 18px to 24px while keeping the visible handle at 6px
• add focused boundary coverage for the expanded hit area and unchanged visual size
• add an issue-specific demo and Bugserver Photo case

Test plan

• rushx test --runInBand __tests__/fill-handle-hit-area.test.ts
• rushx compile
• ESLint for changed TypeScript files
• browser demo verification: hit at 11px, miss at 13px, visual handle 6x6
• pre-push rush test --only tag:package
• Bugserver reproduction before the fix: red FAIL state
• Bugserver branch build 1.0.0.4549: green PASS (hit@11=true, hit@13=false, visual=6x6)

Bugserver case: https://bugserver.cn.goofy.app/case?product=VTable&fileid=6aaf3468168db9005cc382c1&version=1.0.0.4549

Closes #4656

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