Add MIT licence

Declared with the PEP 639 license / license-files fields, so the text ships in
the wheel and the image rather than only the repository. Records the terms of
the two things this repository deliberately does not contain: Boxsim, which is
Visaton freeware and is never redistributed, and kicad-visaton-lib, which is
CC-BY-SA-4.0 and is copied into any image built by tools/build-image.sh.

Verified: pip install exposes License-Expression MIT and packages the text as
dist-info/licenses/LICENSE, both on the host and in the image.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Dockerfile

@@ -17,7 +17,7 @@ LABEL org.opencontainers.image.title="boxsim" \
 
 WORKDIR /opt/boxsim
 
-COPY pyproject.toml README.md ./
+COPY pyproject.toml README.md LICENSE ./
 COPY boxsim ./boxsim
 
 # Staged by tools/build-image.sh: the upgraded Visaton symbol library and the