[repo-assist] perf: use HashSet for culture-code lookup in processFolder

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Summary

Replace Array.distinct + Array.contains (O(n)) with HashSet<string> (O(1)) for the per-directory language-code check in processFolder.

Details

In DocContent.processFolder, every directory traversed checks whether its two-letter name is a known culture code to decide if it is an "other language" subdirectory:

// Before
let allCultures =
    CultureInfo.GetCultures(CultureTypes.AllCultures)
    |> Array.choose (...)
    |> Array.distinct          // O(n2) dedup

let isOtherLang = ... && allCultures |> Array.contains indirName  // O(n) per call

With ~800 culture codes in CultureTypes.AllCultures, each Array.contains call scans up to 800 strings. On a large docs site with many nested directories this adds up.

// After
let allCultures =
    CultureInfo.GetCultures(CultureTypes.AllCultures)
    |> Array.choose (...)
    |> HashSet<string>          // O(1) construction, O(1) lookup

let isOtherLang = ... && allCultures.Contains indirName  // O(1) per call

The Array.distinct step is also removed because HashSet deduplicates automatically.

Test Status

• ✅ dotnet build src/fsdocs-tool/fsdocs-tool.fsproj --configuration Release — passed
• ✅ dotnet fantomas src/fsdocs-tool/BuildCommand.fs --check — no formatting changes required

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