Avoid pattern config split allocations

[Widthdom]

Summary

• Parse bounded pattern config text line-by-line without materializing a whole-file newline split array.
• Keep scalar parsing behavior span-based and allocate only parsed scalar values.
• Add a many-line configured pattern regression test.
• Merge the latest origin/main into fix-issue3052 to resolve the PR conflict and keep the PR diff current.

Validation

• dotnet build
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll . --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter "FullyQualifiedName~SymbolExtractorTests.Extract_ConfiguredPatternYaml|FullyQualifiedName~McpAuditLogTests.ToolsCall_TruncatesAuditRequestId_Issue3237"
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet format CodeIndex.sln --verify-no-changes
• dotnet test

Documentation / Changelog

• Added changelog.d/unreleased/3052.security.md.
• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md updates were made.

Review

• Adversarial review: No blocking/actionable issues found in the final origin/main-based diff.

Fixes #3052