feat: Bitcoin address derivation (segwit, taproot)

[satoshai-dev]

Summary

Derive Bitcoin addresses (segwit p2wpkh, taproot p2tr) from the same mnemonic or private key used for Stacks.

Context

The mock provider already returns placeholder BTC addresses in the Xverse 3-address format (getAddresses). These need to be real, derived addresses for Bitcoin operations to work.

Requirements

• Derive BTC payment address (p2wpkh / segwit) from mnemonic
• Derive BTC ordinals address (p2tr / taproot) from mnemonic
• Use the same derivation paths as Xverse wallet for compatibility
• Update getAddresses response to return real derived BTC addresses
• Update WalletIdentity type to include BTC addresses
• Unit tests for Bitcoin address derivation

Milestone

v0.5