Introduce NsgaiiBuilder and TpeBuilder for samplers

[Alnusjaponica]

Summary

Replace the XxxConfig/from_config construction pattern with builders following the API style of std::thread::Builder: settings are configured by chaining setter methods and the sampler is created with build. Builder::new() is the only entry point, matching thread::Builder::new().

let sampler = NsgaiiBuilder::new()
    .population_size(100)
    .mutation_prob(0.1)
    .crossover_prob(0.9)
    .swapping_prob(0.5)
    .seed(42)
    .build();

let sampler = TpeBuilder::new()
    .n_startup_trials(20)
    .multivariate(true)
    .seed(42)
    .build();

Changes

• Add NsgaiiBuilder and TpeBuilder (with Default) exposing chainable setters and build()
• Remove NsgaiiConfig/TpeConfig and from_config
• Remove the builder() entry point from the samplers in favor of Builder::new() (no dual entry points)
• Keep new, seed_from_u64, and Default on the samplers for backward compatibility; they now delegate to the builders
• Update the pyo3 bindings to construct samplers through the builders
• Rebase onto latest main (the branch previously conflicted with main)

The previous NsgaiiConfig approach is preserved in backup/pr212-before-builder-20260926.

Verification

• cargo test --all-features — 273 tests pass (including new 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

[c-bata]

LGTM!