Add BenchExec run comparison command

[k0lja]

Summary

• add svcomp analyze benchdiff for comparing BenchExec XML files or run directories
• report per-task category/status changes with text, Markdown, and JSON output
• document the new analysis command in the SV-COMP scripts README

Verification

• python3 -m compileall targets/sv-comp/scripts/lib/benchexec_comparison.py targets/sv-comp/scripts/commands/analyze.py
• synthetic BenchExec XML smoke test via compare_benchexec_results, format_benchexec_diff, and diff_to_json

Fixes #40

[k0lja]

Closing this because it was opened while working on the wrong task. The changes address issue #40, but the intended current task is issue #7 (fetch Z3 instead of packaging it). We'll keep issue #7 separate.