docs: verify README examples

[kholdrex]

Summary

Refreshes the README examples so the public snippets compile against the current 0.8 API. Adds integration coverage for the documented examples to keep future API changes from silently drifting away from the README.

Changes

• Updates the install snippet and several README examples for current LSTM, GRU, BiLSTM, linear-layer, training, early-stopping, and scheduler APIs.
• Adds compile-oriented README example tests for early stopping, BiLSTM, GRU, linear layer, and advanced learning-rate scheduling.
• Clarifies that placeholder validation data in examples should be replaced with held-out validation data in real applications.

Test plan

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features
• cargo check --examples