Support SPM execution with process lifecycle management

[ericodx]

Summary

• Add full SPM (Swift Package Manager) execution support with shared sandbox, incremental builds, and escaped child process cleanup
• Implement deferred process cleanup (CapturedOutput + launchCapturingDeferred) to prevent xcresult bundle truncation on Xcode paths while still killing orphan processes on SPM paths
• Reroute schema-excluded mutants to the incompatible executor instead of marking them as unviable, recovering previously lost mutation coverage
• Guarantee SimulatorPool.tearDown() on all error paths, preventing simulator leak across runs
• Add per-mutant narrow exclusion on schematized build failures, replacing the previous per-file exclusion that discarded too many mutants
• Unify IncompatibleTestLaunchResult into TestLaunchResult and centralize Xcode vs SPM result parsing in TestResultResolver

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.

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)