fix(custom): restore emphasis for large datasets (#21442)

[dilipkumar6306316-art]

Problem

Custom series elements with large datasets (~3000+ points) were being marked as silent / ignore
by ECharts for performance reasons. As a result, hover and emphasis (highlighting) did not work
even when emphasis was explicitly defined in renderItem. This is tracked in issue #21442.

Solution

This PR ensures that any custom element with an emphasis state is not marked as silent or ignored.
The fix is applied in src/chart/custom/CustomView.ts just before toggleHoverEmphasis is called.

• Only elements with emphasis are affected
• emphasisDisabled is respected
• Large dataset performance remains unaffected

How to Test

1. Open the custom series example (CodeSandbox or http://localhost:9001)
2. Set dataCount to ~3000-4000
3. Hover over the rectangles
4. Observe that emphasis (highlight) works correctly

Related Issue

Closes #21442

[echarts-bot]

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