Add parallel file processing with rayon

[Zious11]

Summary

Process multiple PCAP files concurrently using rayon for significant speedup on multi-file inputs.

Requirements

• Use rayon (already in dependencies) to parallelize file-level processing
• Each file gets its own Summary + analyzer instances
• Merge per-file results after parallel processing completes
• Add Summary::merge(&mut self, other: Summary) method
• Maintain deterministic output ordering (sort by filename)
• Progress bar should show overall progress across all files

Acceptance Criteria

• rayon::par_iter() over resolved file list
• Per-file analysis with thread-local Summary/analyzers
• Merge step that combines results
• Tests verifying merge produces correct totals
• Benchmark showing speedup on multi-file input