feat: account switching mid-test

[satoshai-dev]

Summary

Allow changing the active wallet identity during a test without reconnecting, enabling multi-user E2E scenarios.

Use Case

Testing two-sided flows: user A supplies STX, user B borrows against it — within the same test.

Requirements

• stacks.wallet.switchAccount(index) or similar API to change active identity
• Re-inject provider with new address/public key into the browser
• Maintain separate nonce tracking per account
• Update StacksFixture type to expose the new method
• Unit tests for account switching logic

Milestone

v0.4