feat: add warmup workflow output

[kholdrex]

Summary

Adds a first-class warmup/burn-in workflow for MCMC samplers so callers can keep discarded warmup states separate from retained posterior draws. The existing sample_with_warmup API remains available for callers that only need retained samples.

Changes

• Adds WarmupMetadata and WarmupRun for warmup counts, retained counts, total iterations, discarded warmup samples, and retained posterior samples.
• Adds Sampler::run_with_warmup while preserving the lightweight sample_with_warmup behavior.
• Covers zero-warmup, zero-retained, retained-only compatibility, and Metropolis-Hastings statistics reset behavior with deterministic tests.

Test Plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps --document-private-items
• cargo bench --no-run --all-features