feat(exchange): add typed order grouping

[drewstone]

Summary

• add a typed OrderGrouping enum for all three Hyperliquid wire values
• expose grouping on bulk orders with and without builder fees
• expose raw asset-index order, cancel, and leverage operations
• preserve existing methods by delegating to the typed/raw methods
• make exchange responses serializable for API passthrough

This supports atomic normalTpsl brackets and numeric asset IDs without accepting arbitrary grouping strings or converting exact decimal strings through floats.

Proof

• cargo fmt --all -- --check
• cargo test --lib: 13 passed, 0 failed
• git diff --check
• clean merge with current master