[si618]

Summary

Fixes the findings from a verify-project review against awesome-humans/dotnet at 15e661e. Test names stay as they are, and tests stay beside their solutions.

• Line endings: Normalises every file to LF without a BOM, and adds .gitattributes. This commit has no content changes, so review the others on their own.
• Testing: Moves both test projects to xunit v3 on Microsoft.Testing.Platform, with the runner opt-in in global.json. Replaces coverlet with Microsoft.Testing.Extensions.CodeCoverage. The console app sets IsTestProject to false, so dotnet test doesn't try to run it.
• Build: Directory.Build.props and Directory.Packages.props follow the templates: artifacts output, latest-recommended analysis, Meziantou.Analyzer, nullable warnings as errors, deterministic builds, and transitive pinning. FSharp.Core goes from 8.0.400 to 10.1.402, the version the SDK ships.
• CI: Pins actions to commit SHAs, takes the SDK from global.json, sets contents: read by default and contents: write only for the benchmark job, and uploads TRX and Cobertura results.
• Code style: Adopts the template .editorconfig, but keeps the repository's var style, PascalCase static readonly fields, and ReSharper settings. Rules that clash with LeetCode's given signatures or the partial Problem class are off, each with a reason: MA0048, MA0016, CA1002, CA1051, and MA0026. Spectre1010 is also off, because Spectre.Console.Analyzer 1.0.0 crashes on it.
• Code fixes: Most come from dotnet format. The hand fixes are culture-aware formatting, a GeneratedRegex partial property with a match timeout, and a single dictionary lookup in TopKFrequent through CollectionsMarshal.GetValueRefOrAddDefault.
• Console fix: Skips cursor moves when output is redirected. Without a terminal, Spectre.Console falls back to Console.SetCursorPosition, and list and benchmark threw ArgumentOutOfRangeException on main.
• Docs: Adds writing-style and agent-attribution rules to AGENTS.md, updates the single-test command for MTP, and drops CLAUDE.md.

Test plan

• dotnet build -c Release from a clean artifacts/ passes with no warnings.

• dotnet format --verify-no-changes passes.

• dotnet test -c Release runs 80 tests, and all pass. TRX and coverage files land in artifacts/test-results.

• dotnet run --project LeetCode -c Release -- benchmark FizzBuzz --csharp completes in a terminal.

• list and benchmark FizzBuzz --csharp succeed with output redirected to a file, and list still moves the cursor in a terminal.