SPM support — SimulatorPool bypass

[ericodx]

Summary

• Phase 4 — SimulatorPool bypass for SPM: MutantExecutor.buildArtifact now uses a switch on projectType. For .spm, it calls BuildStage.buildSPM and returns a real BuildArtifact (with nil xctestrunURL/plist), propagating compilationFailed if the build fails. For .xcode, existing behaviour is preserved (compilation failure returns nil, triggering per-file fallback).
• makePool required no changes — SPM resolves to platform=macOS, which SimulatorManager.requiresSimulatorPool correctly bypasses, creating a macOS slot pool with no simulator cloning.
• SPM schematizable mutants now reach TestExecutionStage and are marked .unviable via the nil-plist guard introduced in Phase 3, until Phase 5 implements SPM test execution.

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)