chore: add LICENSE file — Apache-2.0 or MIT for open source compliance

[BigBen-7]

Description

The repo has no LICENSE file. Open source contributors and integrators need to know the licence terms before using or contributing to the code.

What to do

• Decide on a licence (Apache-2.0 is common for Stellar ecosystem projects)
• Create LICENSE with the full licence text
• Add a licence badge to README.md: [![License: Apache-2.0](https://img.shields.io/badge/License-Apache_2.0-blue.svg)](LICENSE)
• Add license = "Apache-2.0" to Cargo.toml

PR requirement

• Closes #[issue_id] — Timeframe: 24 hours