i18n(ja): restore the pre_split_regions heading so its anchor resolves

[yahonda]

What is changed, added or deleted? (Required)

All nine ja links to #pre_split_regions are currently dead.

The English heading is the identifier itself:

EN: ## pre_split_regions
JA: ## 分割前リージョン {#pre-split-regions}

Translating the heading changed the anchor from pre_split_regions to pre-split-regions, while every link in the ja docs still points at #pre_split_regions. Restoring the identifier as the heading text — and pinning the anchor explicitly — fixes all nine at once.

分割前 was also a mistranslation of the feature: PRE_SPLIT_REGIONS splits the Regions in advance, whereas 「分割前の」 reads as "before splitting".

The same PR also aligns one link's text with its eight siblings:

-[分割前のリージョン](/sql-statements/sql-statement-split-region.md#pre_split_regions)
+[`PRE_SPLIT_REGIONS`](/sql-statements/sql-statement-split-region.md#pre_split_regions)

Seven of the nine links already read [`PRE_SPLIT_REGIONS`]; this was the only one rendered as prose.

How it was checked

• Confirmed #pre_split_regions does not exist as an anchor anywhere in the ja file, while nine links target it.
• After the fix, every sql-statement-split-region.md#… link in the ja docs resolves to a heading that exists.
• The sub-heading ### pre_split_regionsの例 {#examples-of-pre-split-regions} was left untouched — nothing links to it, so changing it would be churn.

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

• master (the latest development version)
• v8.5 (TiDB 8.5 versions)
• v8.4 (TiDB 8.4 versions)
• v8.3 (TiDB 8.3 versions)
• v8.2 (TiDB 8.2 versions)
• v8.1 (TiDB 8.1 versions)
• v7.5 (TiDB 7.5 versions)
• v7.1 (TiDB 7.1 versions)
• v6.5 (TiDB 6.5 versions)
• v6.1 (TiDB 6.1 versions)
• v5.4 (TiDB 5.4 versions)
• v5.3 (TiDB 5.3 versions)

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

• This PR is translated from: N/A (Japanese-only fix)
• Other reference link(s): sibling ja PRs i18n(ja): add missing object-marker particle (wo) before code-span transitive verbs #23272 i18n(ja): add missing particle after Markdown links (wo before verbs, wa for subject) #23273 i18n(ja): miscellaneous review fixes (bit-shift description) #23274 i18n(ja): remove stray digits inserted before code spans #23275 i18n(ja): standardize the "Data Service" product name (データサービス → Data Service) #23291 i18n(ja): restore admonition labels to English (Note/Warning/Tip) #23292 i18n(ja): fix trailing "page number" typo after page reference links #23294 i18n(ja): restore variable names and clean up heading anchors in system variable docs #23295 i18n(ja): unify "plan cache" katakana term (remove stray space) in sql-prepared-plan-cache.md #23297 i18n(ja): fix partial negations rendered as full negations #23311 i18n(ja): restore text dropped by the translation pipeline #23315 i18n(ja): restore text dropped by the translation pipeline (part 2) #23316 i18n(ja): restore particles dropped after code spans and links #23318 i18n(ja): repair sentences scrambled by the translation pipeline #23320 i18n(ja): restore particles dropped before nouns and copulas #23322

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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