Introduce PedAnovaBuilder for PedAnovaImportanceEvaluator

[Alnusjaponica]

Summary

Replace the positional constructor arguments of PedAnovaImportanceEvaluator with a builder following the API style of std::thread::Builder: settings are configured by chaining setter methods and the evaluator is created with build (companion of #212 for samplers).

let evaluator = PedAnovaImportanceEvaluator::builder()
    .target_quantile(0.2)
    .region_quantile(0.9)
    .evaluate_on_local(false)
    .n_steps(100)
    .prior_weight(2.0)
    .min_n_trials_in_regime(3)
    .build()
    .unwrap();

Changes

• Add PedAnovaBuilder (with Default) exposing builder(), chainable setters, and build()
• The builder exposes all evaluator settings: target_quantile, region_quantile, evaluate_on_local, n_steps, prior_weight, and min_n_trials_in_regime, which were previously fixed to their internal defaults
• new and Default are kept for backward compatibility and now delegate to the builder
• The validation (0.0 < target_quantile < region_quantile <= 1.0) moved into build
• Update the pyo3 binding to construct the evaluator through the builder

Verification

• cargo test --all-features — 274 tests pass (including a new test_builder and 3 doctests)
• cargo test -p rustuna_storage --locked -- --ignored (from rustuna_pyo3) — 3 pass
• cargo clippy --locked --workspace --lib --bins --tests --examples --all-features -- -D warnings — clean
• cargo fmt --all -- --check — clean
• Python: uv run --no-sync pytest tests/ — 701 pass (constructor, ValueError, and warning behavior verified through the new builder path)