Add configurable preprocessing steps

[cmccomb]

Summary

• add reusable scaling, imputation, categorical encoding, power transform, and column filtering steps with persisted state
• expose matching configuration structs under settings and document the new capabilities with AutoGluon/caret style examples
• add integration tests that exercise each preprocessing step and their composition

Testing

• cargo fmt
• cargo clippy --all-targets -- -D warnings -D clippy::pedantic
• cargo test

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Codex Task