Change license to GPLv3

[EmanFateen]

Summary

• Replace the MIT license text with GNU GPL version 3.
• Update package metadata and the lockfile root package entry to use GPL-3.0-only.
• Update the README license note to say the project is GPLv3 licensed.

Why GPL-3.0-only instead of GPL-3.0

GPL-3.0-only is the current SPDX identifier for code licensed under exactly GPL version 3. The older GPL-3.0 identifier is deprecated because it does not make the “only this version” intent as explicit. Since this change uses the GPLv3 license text without adding the “or any later version” option to the project metadata, GPL-3.0-only is the precise npm/SPDX value.

If the project should allow GPLv3 or later instead, the correct SPDX identifier would be GPL-3.0-or-later.

Validation

• Confirmed package.json and the root package-lock.json package entry both parse and report GPL-3.0-only.
• npm test could not start in the existing Docker container because Rolldown's native optional dependency is missing from node_modules (@rolldown/binding-linux-arm64-musl).