BTC Addresses for Stacks Transactions

Proposes allowing Stacks transactions (notably sBTC) to use native Bitcoin address formats (e.g. bc1…) instead of Stacks SP… addresses. The goal is to reduce UX friction for Bitcoin users by aligning address formats with what they already recognize, while preserving existing Stacks address semantics under the hood. This is primarily a tooling and ecosystem standard rather than a consensus-breaking change.