Add mutation testing module for test quality assessment

[Copilot]

Implements a mutation testing system that inserts bugs into code, runs tests against mutated versions, and reports which mutations survive—indicating gaps in test coverage.

Implementation

• MutationTestingService: Core service providing 5 mutation operators (ARITHMETIC, COMPARISON, LOGICAL, UNARY, RETURN) with 21 mutation types. Generates mutants, executes tests, calculates mutation scores.

• IPC Integration: 5 handlers expose mutation testing to renderer process (mutation:getOperators, mutation:generateMutants, mutation:runTests, mutation:getReport, mutation:reset)

• Test Coverage: 23 tests covering mutant generation, test execution, score calculation, and edge cases

Usage

const { MutationTestingService } = require('./src/main/mutation-testing.js')
const service = new MutationTestingService()
service.setProjectPath('/path/to/project')

const mutants = await service.generateMutants('src/math.js', ['ARITHMETIC'])
const report = await service.runMutationTesting('npm test')

console.log(`Score: ${report.summary.mutationScore}%`)
console.log(`Killed: ${report.summary.killedMutants}/${report.summary.totalMutants}`)

From Electron renderer:

const { report } = await window.api.invoke('mutation:runTests', {
  filePath: 'src/math.js',
  testCommand: 'npm test'
})

Documentation

• docs/MUTATION_TESTING.md: Complete API reference, mutation operators, interpretation guide
• examples/mutation-testing-example.js: Working demonstration with sample code

Original prompt

Add mutation testing support; Puffin should have a module that inserts bugs, runs tests, and count killings

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