Adds a finishing phase that runs after autotuning completes to simplify the found configuration. The phase iteratively resets parameters to their default values, keeping simplifications that don't degrade performance.

This produces cleaner, more minimal configs by removing unnecessary customizations while maintaining the same performance.

• New run_finishing_phase() method in PopulationBasedSearch
• New Config.minimize() method to strip default values from configs
• Configurable via finishing_rounds parameter (default: 3) or HELION_AUTOTUNE_FINISHING_ROUNDS env var
• Enabled for PatternSearch, LFBOPatternSearch, and DifferentialEvolution