feat: granular test cache invalidation

[ericodx]

Summary

• Replace monolithic test file hash with per-file SHA256 hashing for granular cache invalidation
• Add KillerTestFileResolver to map test names to file paths (XCTest and Swift Testing formats)
• Invalidate cache entries selectively based on which test files changed, preserving valid entries
• Extract test helpers from 19 test suites into shared TestSupport fixtures
• Remove dead code identified by Periphery scan

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

Which stages are affected?

• TestExecutionStage
• IncompatibleMutantExecutor
• PerFileBuildFallback
• CacheStore
• Models / RunnerInput contract

New Types

Type	Responsibility
TestFileDiff	Tracks added/modified/removed test files between runs
KillerTestFileResolver	Maps test names to file paths for targeted invalidation
CacheMetadata	Persists per-file test hashes between runs

Invalidation Rules

Status	Behavior
unviable, killedByCrash	Always preserved (independent of test changes)
killed(by:) with known killer file	Invalidated only if killer test file changed
killed(by:) without killer file	Conservatively invalidated
survived, noCoverage, timeout	Always invalidated when any test file changed

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) — 615 tests, 69 suites