Introduce ProjectType and SPM project detection

[ericodx]

Summary

• Introduces ProjectType enum (xcode(scheme:destination:) / spm) as the single source of truth for project kind throughout the pipeline
• Replaces the loose scheme: String / destination: String fields in RunnerConfiguration.BuildOptions, DiscoveryInput, and RunnerInput with a single projectType: ProjectType value
• ConfigurationResolver now auto-detects SPM projects: when neither --scheme nor --destination is provided and a Package.swift is found at the project path, it resolves to .spm
• ProjectDetector and ConfigurationFileWriter updated with SPM awareness — init generates SPM-specific config when a Package.swift is detected
• Xcode projects (XCTest and Swift Testing) continue to work exactly as before — this phase is purely additive

Type of Change

• feat: A new feature has been added.
• 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

• BuildStage
• IncompatibleMutantExecutor
• CLI / Configuration
• Models / RunnerInput contract

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)
• All tests pass locally (swift test)