[repo-assist] fix: Seq.meanTruncatedBy trims both ends

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🤖 This is an automated PR from Repo Assist.

Root cause: Seq.meanTruncatedBy skipped k values from the low end but then took n - k elements, so the high end was never trimmed. It also used floor, whereas meanTruncated truncates via int. Its documented example result (7.0) was also wrong.

Fix: use Seq.take (n - 2 * k) so both ends are trimmed, matching meanTruncated. The doc example is corrected to 6.0 (keeps 2,3,4, doubled mean).

Tests: new Seq.meanTruncatedBy tests cover both-end trimming, equivalence with meanTruncated, and a zero proportion.

Behaviour change: meanTruncatedBy results change for proportion > 0. This is a bug fix, but maintainers should be aware.

Test Status

• Build (Release): ✅
• dotnet test --filter meanTruncated: ✅ 10/10. The Seq filter: ✅ 359/359.
• Full ./build.sh RunTests was not run.

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