Fix console output synchronization

[Widthdom]

Summary

• Synchronize Console.Out and Console.Error at CLI startup with TextWriter.Synchronized.
• Ensure spinner startup also synchronizes captured/redirected writers before fallback output.
• Add console regression coverage for concurrent whole-string writes and redirected spinner output.

Validation

• dotnet build CodeIndex.sln
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~ConsoleUiTests
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter ConsoleUiTests --no-restore (adversarial review)
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index . --commits fd8f40485bfd82f920e925799cc6ef53e0a9fbb9 --json

Full dotnet test CodeIndex.sln was attempted but failed in existing Rust associated-type-default coverage (SymbolExtractorTests.Extract_RustTraitAssociatedTypeDefaults_RecordsPropertySymbols) and then did not terminate promptly; open related issues already exist: #2603, #2609, #2611.

Documentation and Changelog

• Added changelog fragment: changelog.d/unreleased/1734.fixed.md
• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes.

Follow-up Candidates

• Existing Rust associated type default failures are tracked by Rust associated type defaults are not emitted for trait symbols #2603, Rust associated type defaults are skipped under trait containers #2609, and Rust associated type defaults are skipped under protocol-kind traits #2611.

Fixes #1734