feat(alpha): evaluate deterministic Ridge and CART baselines

[proerror77]

Change contract

Fit deterministic Ridge and depth-bounded shallow CART baselines from one immutable Factor Bank using training-only folds and validation-only scoring.

Issue relationship

Refs #600

Out of scope

Mission persistence, MCTS admission, Factor-Bank subset search (#601), ACK/OSS execution, and Paper/Shadow/Live.

Dependencies and merge order

Stacked on #662. Merge after #662 and before the producer and MCTS admission PRs.

Focused validation

• cargo test -p alpha-engine --locked (83 passed, 1 credential-gated LLM test ignored)
• cargo clippy -p alpha-engine --locked -- -D warnings
• cargo fmt -p alpha-engine -- --check
• Constant-column Ridge and stable CART threshold/tie-breaking counterexamples pass.

Rollout and rollback

Research-only code. Roll back this commit before dependent producer/admission PRs.

Scope exception

None.

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 9b65bf0d-9732-4c72-8fb2-c247ab517ab6

📥 Commits

Reviewing files that changed from the base of the PR and between a0f2191 and cc8a0b0.

📒 Files selected for processing (5)

• rust_hft/alpha-harness/engine/src/baselines.rs
• rust_hft/alpha-harness/engine/src/engines/bayesian.rs
• rust_hft/alpha-harness/engine/src/engines/mod.rs
• rust_hft/alpha-harness/engine/src/formula_evaluator.rs
• rust_hft/alpha-harness/engine/src/lib.rs

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