test: vitest testing setup

[robertraf]

This pull request introduces a comprehensive testing setup using Vitest, adds initial unit tests for core game logic and data helpers, and integrates test coverage reporting into CI workflows. The changes also update the project configuration to support Vitest and coverage, and document new testing commands.

Testing infrastructure and configuration:

• Added Vitest as the testing framework, along with coverage support via @vitest/coverage-v8. Updated vite.config.ts to use vitest/config and configured test environment and file inclusion. [1] [2] [3] [4]
• Introduced new npm scripts for running tests, watching tests, and generating coverage reports in package.json.

CI integration:

• Updated GitHub Actions workflows (deploy.yml and pr-preview.yml) to run tests with coverage as part of the CI process. [1] [2]

Unit tests for core logic:

• Added tests for the game context reducer, including state loading, crafting, and battle rewards, in GameContext.reducer.test.ts. Exposed internal utilities for testing purposes. [1] [2]
• Added tests for battle data helpers, including wave config scaling, equipment bonuses, and deterministic rewards, in battleData.test.ts.
• Added tests for item data helpers, including artifact and equippable item identification, and item definition consistency, in gameData.test.ts.

Documentation:

• Updated README.md to document new test commands (npm run test, npm run test:watch).

[github-actions]

Preview deployment

URL	https://pr-19.spacecraft-50l.pages.dev
Commit	f522d1c
Status	Ready

Deployment updated on every push to this PR.