Add --top N flag to limit displayed files

[MatthewMckee4]

Summary

• Adds --top N CLI flag to show only the top N files by line count
• Truncates file list in table and JSON output while keeping totals reflecting the full count
• --top is a no-op with --summary since there is no per-file list

Closes #2

Test plan

• top_truncates_table_output — top 2 of 3 files shown, totals reflect all 3
• top_truncates_json_output — JSON has 2 files but full totals
• top_is_noop_for_summary — summary unaffected by --top
• top_zero_shows_no_files — --top 0 shows no files, totals intact
• top_larger_than_file_count_shows_all — --top 100 with 2 files shows all

[codecov]

Codecov Report

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