Bengali and Assamese characters showing improperly after following #680 to use PC fonts.

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Include at least:
1. Steps to reproduce the issue (e.g. the command you ran)
2. The unexpected behavior that occurred (e.g. error messages or screenshots)
3. The environment (e.g. operating system and version of manim)

### If this is a feature request:
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此 issue 參考了先前關於 PC 字型的修正 (#680)。先從檢查 manim 中的字型處理程式碼開始，可能位於與文字渲染或設定相關的模組中。查看 Bengali 和 Assamese 字元是如何處理的。使用範例文字進行測試以查看不當渲染。檢查問題是否在字型選擇、字形映射或渲染管線中。

由索引模型根據 Issue 內容生成。