Add LICENSE file

[dblizinski]

Changing license from AGPL to
Apache

Summary

Licensing layer

• Apache-2.0 extension (3D Slicer, wellness/veterinary module, partner SDK, language binding)
• AGPL-3.0 core (firmware, sensing/beamforming/reconstruction/signal-processing/calibration algorithms, hardware reference design)
• Docs / data / tooling only
• This PR moves a file between the AGPL core and the Apache layer

Checklist

• Commits are signed off (DCO): git commit -s
• New files carry the correct SPDX-License-Identifier for their layer
• Tests and docs updated
• No incompatible third-party code introduced
• CI is green and CODEOWNERS review requested

Notes for reviewers

[ebrahimebrahim]

(do not worry about the unrelated failing tests, will be fixed in #501 and then I will rebase this)