add block mode multicast e2e coverage

[packethog]

Summary

• add CI-safe block-mode multicast e2e coverage using reduced real Hyperliquid validator by-block fixtures
• capture TOB and DoB UDP output with in-process loopback collectors and compare normalized golden packet bytes
• version the reduced source inputs, derived fixture data, goldens, manifest, and regeneration scripts
• add a GitHub Actions workflow for clippy and workspace tests

Validation

• python3 -m py_compile server/tests/fixtures/hl_block_mode/generate_from_source.py server/tests/fixtures/hl_block_mode/regenerate_from_host.py
• server/tests/fixtures/hl_block_mode/regenerate_from_host.py --help
• cargo test -p server listeners::order_book::block_mode_multicast_e2e -- --nocapture
• cargo clippy --workspace --all-targets
• cargo test --workspace
• workflow YAML parse check