SFT-7483: Preserve master private HD key bytes during UR decoding

[Jacksper13]

Summary

Fix master-private hdkey decoding so it validates the required zero prefix and copies all 32 secret-key bytes from the 33-byte BIP32 key-data field.

The previous decoder copied bytes 0..32, retaining the prefix and dropping the final secret-key byte while returning success.

Changes

• require the master private key-data prefix to be zero
• decode the secret from bytes 1..33
• verify exact round trips through both hdkey and crypto-hdkey generic Value routes
• reject nonzero prefixes and key-data fields that are not exactly 33 bytes