BIP-32 key derivation modulo operation may produce invalid keys

Problem

The BIP-32 key derivation uses a simple modulo operation that may not correctly handle edge cases per the BIP-32 specification.

Location

src/protocol/account.ts:209

Current Code

finalKey = childKeyBigInt % curveOrder;

Concern

According to BIP-32 spec:

• If the resulting key is zero or >= curve order, the child key is invalid
• Should skip to next index rather than use modulo
• Modulo could produce keys with different security properties

Recommendation

1. Verify against BIP-32 test vectors
2. Implement proper invalid key handling:

if (childKeyBigInt === 0n || childKeyBigInt >= curveOrder) {
  // Invalid key - derive next index per BIP-32 spec
  return this.deriveChild(index + 1);
}

Priority

Medium - Edge case but security-sensitive

Start at src/protocol/account.ts:209 and trace the child-key derivation flow around the modulo operation. Compare the behavior with the BIP-32 specification and test vectors, then verify the invalid-key handling for zero and values at or above the curve order. Done means the edge cases are handled according to the specification and the relevant derivation tests pass.