Add --durations N flag

[MatthewMckee4]

Summary

• Add a --durations N CLI flag that displays the N slowest tests after a run completes
• Aggregates per-worker duration data (already cached in durations.json) into AggregatedResults and prints a sorted summary
• Supports edge cases: N=0 shows header only, N > test_count shows all tests, works alongside failing tests

Closes #466

Test plan

• durations_shows_slowest_tests — verifies top-N sorting with sleep-based timing
• durations_shows_all_when_n_exceeds_test_count — verifies graceful handling when N > total tests
• durations_zero_shows_header_only — verifies N=0 edge case
• durations_with_failing_tests — verifies durations display alongside failure diagnostics
• All 681 existing tests pass
• Pre-commit checks pass

🤖 Generated with Claude Code

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 1 untouched benchmark

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_{Comparing add-durations-flag (a56b66c) with main (80ccaf5)}