fix: reject empty routing batches

[jamilahmadzai]

Closes #124

Bounty: GrantFox OSS campaign for issue #124

Summary

• add RouterError::EmptyBatch with code 19
• reject empty register_pairs and set_pair_fees_bps batches after pause/admin checks and before max-size validation
• update README/API docs and batch tests while keeping single-entry batches supported

Validation

• cargo fmt --all -- --check
• cargo build
• cargo test test_register_pairs_rejects_empty_batch -- --nocapture
• cargo test test_set_pair_fees_bps_rejects_empty_batch -- --nocapture
• cargo test test_batch -- --nocapture
• cargo test -- --nocapture
• git diff --check