[docs](topn) Document phase-2 File Cache no-write option

[bobhan1]

Versions

• dev
• 4.x
• 3.x
• 2.1 or older (not covered by version/language sync gate)

Languages

• Chinese
• English
• Japanese candidate translation needed

Docs Checklist

• Checked by AI
• Test Cases Built
• Updated required version and language counterparts, or explained why not
• If only one language changed, confirmed whether source/translation counterparts need sync

Summary

• document the enable_topn_lazy_mat_phase2_no_write_file_cache session variable in the TOPN optimization guide
• explain full local hits versus remote-only cache-miss behavior
• clarify that the option applies only to TOPN lazy-materialization phase 2 for Doris internal tables in storage-compute separation mode
• add a runnable SQL example, aggregate and per-BE Profile metrics, verification guidance, caveats, and tuning recommendations

Related changes

• [improvement](topn) Add option to skip file cache writes in topn lazy materialization doris#65021
• branch-4.0: [improvement](topn) Add option to skip file cache writes in topn lazy materialization doris#66172
• branch-4.1: [improvement](topn) Add option to skip file cache writes in lazy materialization doris#66414

Scope

Synchronize the current and 4.x TOPN optimization guides in Chinese, English, and Japanese. Versions 3.x and 2.1 are intentionally out of scope because the corresponding code change is available on master and branch-4.0.

Validation

• git diff --check
• python3 scripts/check_md_links_single.py <file> --no-external for all six synchronized pages
• front matter JSON, Markdown fence, admonition balance, and cross-version consistency checks