feat: add CKD support (derivation_path + client-side key derivation)

[ImmuneFOMO]

What
Added derivation_path support to signTransaction() and a new ckdutil module for client-side BIP-32 non-hardened child key derivation.
The Go server already supports CKD at signing time (pkg/mpc/ckd.go, pkg/ckdutil/child_derivation.go), but the TS client had no way to pass derivation_path or derive child keys locally.

Changes

Signing:

• derivation_path?: number[] added to SignTxMessage type
• derivationPath param added to signTransaction() - passes through to wire message
• Excluded from signature payload (matches Go Raw() behavior)
  ckdutil module (new, mirrors Go pkg/ckdutil):
• deriveSecp256k1ChildCompressed() - BIP-32 non-hardened derivation on secp256k1
• deriveEd25519ChildCompressed() - same for ed25519
• deriveEthereumAddress() - convenience: derivation + keccak256 address
• compressPublicKey() - 64/65-byte uncompressed → 33-byte compressed
  Tests:
• Unit tests for both curves, including BIP-32 Test Vector 2 verification
• SignTxMessage payload assertions (derivation_path present/absent)
  Deps:
• @noble/curves, @noble/hashes (runtime)
• vitest (dev)

Use case

Enables HD wallet architecture: one master wallet via DKG, user wallets derived by index at signing time. No per-user keygen needed, a single backup at init time is enough to restore all wallets if the server dies.

[ImmuneFOMO]

Hello, @anhthii , would love to get your thoughts on this PR