bench(csharp-query): Summarize C# query source registrations in sweep reports

[s243a]

Summary

• extend benchmark_csharp_query_source_mode_sweep.py to parse source_registration_* metrics
• add source_registrations_by_mode to TSV and Markdown report output
• make dependency depth runners emit C# query metrics rows consistently
• update benchmark README to document source-registration storage summaries

Validation

• git diff --check
• python3 -m py_compile for touched report/dependency runners
• traced TSV smoke for category-influence with auto,artifact-prebuilt
• traced Markdown smoke for dependency-depth with auto,preload
• non-trace TSV smoke for dependency-depth with auto,preload