i18n(ja): translate the untranslated Scattering glossary term to match Splitting

[yahonda]

What is changed, added or deleted? (Required)

tidb-lightning/tidb-lightning-glossary.md's ### Scattering heading, and the inline word "Scattering" in its body, were left untranslated — unlike every other entry in this glossary, including the directly analogous Region operation ### Splitting, which is properly translated to ### 分割.

Since Scatter/Split/Merge are all operations performed on a Region, and Splitting is already translated, translate Scattering to 散布 for consistency:

• ### Scattering {#scattering} → ### 散布 {#scattering} (anchor unchanged)
• ...再割り当てする操作。Scatteringにより、... → ...再割り当てする操作。散布により、...

(Superseded PR #23878 took the opposite approach — making other 散布リージョン sites literal English on the assumption that the glossary's "Scattering" was an intentional literal-English decision. Further review showed it was actually just this untranslated leftover, so #23878 was closed in favor of this fix.)

Which TiDB version(s) do your changes apply to? (Required)

• i18n-ja-release-8.5 (TiDB Japanese documentation for TiDB 8.5 versions)

What is the related PR or file link(s)?

• This PR is translated from: N/A (Japanese-only fix for a leftover untranslated term, no corresponding English-source change)

AI agent involvement

• The changes in this PR were primarily made by an AI agent on behalf of the PR author.

Do your changes match any of the following descriptions?

• Delete files
• Change aliases
• Need modification after applied to another branch
• Might cause conflicts after applied to another branch

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• tidb-lightning/tidb-lightning-glossary.md

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