SPM support — fixture, integration tests, and additionalEnvironment API

[ericodx]

Summary

• Add CalcLibrary SPM fixture (Package.swift + Calculator, Validator, Logic sources + XCTest suite with deliberate partial coverage)
• Add MutantExecutorSPMIntegrationTests: end-to-end SPM pipeline producing known killed/survived sets (killed={m1,m2,m4}, survived={m3,m5,mi1})
• Fix TestExecutionStage.launchSPM environment merge — previously replaced the entire process environment, causing swift test to fail without PATH/HOME
• Add additionalEnvironment: [String: String] parameter to ProcessLaunching/ProcessLauncher: merges into the inherited parent environment rather than replacing it; SPM schematized path uses this to inject __SWIFT_MUTATION_TESTING_ACTIVE
• Add SPM crash fixture tests for TestOutputParser covering Fatal error: and EXC_BAD_INSTRUCTION patterns from real swift test output

Type of Change

• feat: A new feature has been added.
• fix: A bug has been fixed.
• perf: A code change that improves performance.
• refactor: A code change that neither fixes a bug nor adds a feature.
• test: Addition or correction of tests.
• docs: Changes only to the documentation.
• ci: Changes related to continuous integration and deployment scripts.
• build: Changes that affect the build system or external dependencies.
• deps: Dependency version upgrades with no logic changes.
• chore: Other changes that do not fit into the previous categories.
• revert: Reverts a previous commit.

Invariants Checklist

• Original project is never modified — all mutations happen inside an isolated sandbox
• xcodebuild build-for-testing runs exactly once for all schematizable mutants
• No mutant results are lost or duplicated
• Mutant positions (file, line, column) are accurate in all reported results
• A cancelled task never leaves a simulator slot permanently acquired from the pool
• schematizedContent never contains the __swiftMutationTestingID global declaration
• Swift 6 Strict Concurrency compatible
• Pipeline stages remain stateless pure transformations

Pipeline Impact

Which stages are affected?

• SandboxFactory
• BuildStage
• SimulatorPool
• TestExecutionStage
• IncompatibleMutantExecutor
• PerFileBuildFallback
• CacheStore
• Reporters (Text / JSON / HTML / Sonar)
• CLI / Configuration
• Models / RunnerInput contract
• None

Testing

• Unit tests added or updated
• Tests use mock ProcessLaunching — no real xcodebuild, xcrun simctl, or xcresulttool
• Tests use FileHelpers for any filesystem interaction (temp directories only)
• Snapshot tests added or updated (if reporter output format changed)
• Integration tests added or updated and tagged separately (if pipeline or CLI behavior changed)
• All tests pass locally (swift test)