Refactor large services into focused partial classes

[Widthdom]

Summary

Split several oversized service files into focused partial classes without changing behavior.

This refactor:

• extracts ProgramRunner constructor/constants into Runner/ProgramRunner.Core.cs
• moves IL comparison/filtering helpers into Services/ILOutputService.Comparison.cs
• moves Markdown report helper logic into Services/ReportGenerateService.Helpers.cs
• extracts HTML semantic change rendering into Services/HtmlReport/HtmlReportGenerateService.SemanticChanges.cs
• simplifies shared HTML detail row rendering across inline diffs, semantic changes, and dependency changes
• updates doc/DEVELOPER_GUIDE.md and CHANGELOG.md to match the new file layout

Why

Some core services had grown beyond the repository's preferred file-size limits, which made navigation and maintenance harder.

This change keeps the existing call paths and behavior intact while making responsibilities easier to locate:

• orchestration stays in the main partial
• comparison/rendering helpers move into dedicated partials
• shared HTML detail rendering becomes easier to reuse consistently

Validation

• dotnet test FolderDiffIL4DotNet.Tests/FolderDiffIL4DotNet.Tests.csproj --configuration Release --nologo
• Result: 1919 passed, 1 skipped