Allow indexer reads during concurrent source writes

[Widthdom]

Summary

• Route indexer source content and checksum reads through BoundedFile.OpenReadForIndexContent with FileShare.ReadWrite | FileShare.Delete.
• Add regression coverage for reading and checksumming source files while a writer keeps the file open.
• Document the source-read sharing contract and add the Audit file open, stream, and directory creation policy for bounds, sharing, and permissions #4078 changelog fragment.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedNameFileContentLoader_Load_AllowsConcurrentWriterShare_Issue4078|FullyQualifiedNameTryComputeChecksum_FilePathAllowsConcurrentWriterShare_Issue4078" -p:UseSharedCompilation=false
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --no-build --filter "FullyQualifiedNameFileContentLoader_Load_AllowsConcurrentWriterShare_Issue4078|FullyQualifiedNameTryComputeChecksum_FilePathAllowsConcurrentWriterShare_Issue4078"
• dotnet build -p:UseSharedCompilation=false
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json

Full Release dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build was stopped after unrelated performance guard failures; tracked separately in #4087.

Documentation and changelog

• DEVELOPER_GUIDE.md
• changelog.d/unreleased/4078.fixed.md

Review

• Codex adversarial review: No blocking/actionable issues found.
• codex exec review attempt was blocked by a Codex usage limit, so the local Codex review followed .codex/workflows/adversarial-review.md.

Follow-up candidates

• Performance guard tests exceed local Release budgets #4087

Fixes #4078