fix: correct BLS key derivation logic and improve test coverage (#113)

* fix: correct BLS key derivation logic and improve test coverage

* test: add comprehensive BLS HD derivation tests matching C++ implementation

- Add long derivation path test (m/(2^31+5)/0/0/(2^31+56)/70/4)
- Add serialization roundtrip tests for ExtendedBLSPrivKey and ExtendedBLSPubKey
- Add exact C++ test vectors for compatibility verification
- Add legacy HD compatibility tests
- Add extended unhardened derivation tests
- Refactor hardened index creation to use from_hardened_idx() method
- Ensure full test parity with C++ bls-signatures library

All 16 BLS BIP32 tests passing with complete coverage

* style: apply cargo fmt

Author: PastaPastaPasta