feat: multi-account fixtures from single mnemonic

[satoshai-dev]

Summary

Configure multiple wallet identities from a single mnemonic, each with its own address and key pair.

Context

The library already supports accountIndex for mnemonic derivation, but only one account is active per test. This feature enables pre-configuring multiple accounts and switching between them.

Requirements

• testWithStacks({ mnemonic, accounts: [0, 1, 2] }) or similar config
• Each account gets its own WalletIdentity (address, publicKey, privateKey)
• Expose stacks.wallet.accounts array alongside stacks.wallet.address
• Test dApp multi-account page + E2E spec showing two users interacting
• Unit tests for multi-account derivation

Milestone

v0.4