Decide the first implementation stack

[bt1142msstate]

Goal

Choose the initial implementation shape for PayRails.

Options To Evaluate

• Plain TypeScript package.
• Framework-neutral core plus optional Firebase adapter.
• Firebase-first application.

Acceptance Criteria

• Explains the recommended stack and tradeoffs.
• Accounts for open-source reuse, security, testing, and future hosted deployment.
• Produces a decision record before application code is scaffolded.