test: bound early stopping example training

[kholdrex]

Summary

Bounds the early stopping example so example-backed regression tests stay fast and deterministic. The demo now uses small named training budgets and fixture sizes while preserving coverage for validation-loss monitoring, training-loss monitoring, disabled weight restoration, and higher-patience configuration.

Changes

• Add bounded early-stopping demo configuration helpers and fixture-size constants.
• Reduce the synthetic early-stopping fixture to deterministic, small training and validation sets.
• Extend example training-bound regression tests to cover early stopping configs, patience limits, fixture sizes, and deterministic fixture generation.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo run --example early_stopping_example