Remove unused mutation_rate field from MutationSettings

[Copilot]

MutationSettings had a mutation_rate: f32 field that was never read anywhere — pure dead code. It was declared in the struct and initialized in Default but had no effect on any mutation logic.

Changes

• Removed pub mutation_rate: f32 from MutationSettings
• Removed mutation_rate: 0.01 from MutationSettings::default()

Original prompt

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This section details on the original issue you should resolve

<issue_title>Forgot to delete old mutation_rate field on MutationSettings</issue_title>
<issue_description>oopsies. it does absolutely nothing. surprised it wasn't caught for deadcode. oh well, i'll make a patch later addressing this.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Forgot to delete old mutation_rate field on MutationSettings #112

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[Copilot]

Pull request overview

This PR removes dead code from the MutationSettings struct by deleting the unused mutation_rate field that was never referenced anywhere in the codebase. The mutation rate is passed as a parameter to mutation methods rather than being stored in the settings struct.

Changes:

• Removed unused mutation_rate: f32 field from MutationSettings struct
• Removed initialization of mutation_rate: 0.01 from MutationSettings::default()

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