docs: add MIT LICENSE file

[atani]

Adds the MIT LICENSE file. The README already advertises this project as MIT-licensed (License badge), but the repository had no LICENSE file, which blocks adoption and shows "No license" / "License not recognized" on the repo page.

Why

• A missing LICENSE is a common blocker for users who want to depend on or contribute to a project.
• GitHub displays the license in the sidebar and in search facets once a recognized LICENSE file exists, improving discoverability.

Test plan

• LICENSE content is the standard MIT text (OSI-approved), matching the README badge.
• Copyright holder/year aligned with the author other repos (Akira Taniwaki, 2026).