i18n(ja): restore literal English Grafana panel/series names in TiDB Lightning monitoring doc

[yahonda]

What is changed, added or deleted? (Required)

tidb-lightning/monitor-tidb-lightning.md documents the TiDB Lightning Grafana dashboard (Rows 1-7). Its Panel and Series table-column values, and the English row-name portion of each ### N行目: <name> heading, had been translated into Japanese. Since these are literal panel/series/row labels from the real Grafana dashboard, translating them makes it impossible for a reader to match the doc against what they actually see in the Grafana UI.

Verified against the real TiDB Lightning dashboard JSON (pingcap/monitoring) and the parallel English doc file. Restored to literal English across all 7 rows, for example:

• インポート速度 → Import speed, Chunk処理期間 → Chunk process duration
• アイドルワーカー → Idle workers, 外部リソース → External resources
• SQL処理速度 → SQL process rate, 合計バイト数 → Total bytes
• Series values such as クローズ済みエンジン → closed-engine, 応募者 → apply worker, ストレージサイズ / 3 → storage_size / 3
• Row headings, e.g. 1行目: スピード → 1行目: Speed, 7行目: インポート速度 → 7行目: Import speed

Also translated one Description cell (Row 1, "Chunk process duration") that had been left as raw, untranslated English ("Average time needed to completely encode one single data file") into Japanese.

Description-column prose (the actual explanatory text) was left in Japanese as before — only the literal panel/series/row-name labels were restored to English.

Follow-up scope added to this PR (same file, same "keep literal identifiers literal" principle, found during review):

• The ## Monitoring metrics section's Prometheus label keys type/name/state/result had also been translated (タイプ/名前/状態/結果), even though these are the actual label keys used in the real metric output (e.g. lightning_importer_engine{type="open"}). Restored to literal English.
• Unified the state label's description wording (テーブルの状態/エンジンの状態 → テーブルのステータス/エンジンのステータス) to match this file's own dominant rendering of "status" (used in every metric's intro sentence and in lightning_chunks's own state description), plus the same word in the estimated caveat.
• Translated the lightning_import_seconds description, which had been left as raw, untranslated English.

Deferred to the upcoming EN→JA sync rather than hand-applied here: unifying the lightning_idle_workers label descriptions ("Xの余り") with this document's own Grafana panel table wording ("未使用の数はX"), since the underlying English-source fix (#23881) hasn't merged yet.

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 restoring literal dashboard labels, no corresponding English-source change)
• Other reference link(s): tidb-lightning: unify idle-worker wording with the Grafana panel table #23881 (companion English-source wording fix)

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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