Add code coverage reporting and badge

[gauthierpiarrette]

Problem
No test coverage measurement exists. Contributors can't see which code paths are untested, and there's no coverage badge in the README to signal project quality.

Proposed solution

• Add pytest-cov to dev dependencies
• Add --cov=src/mcpplay --cov-report=xml to the CI test step
• Upload to Codecov or Coveralls (free for OSS)
• Add coverage badge to README
• Set a minimum threshold (e.g., 80%) that fails CI if coverage drops

Alternatives considered
Coverage without a badge. The badge in the README is what signals quality to potential users and contributors at first glance.