Suite-granularity shard enumeration is byte-stable

Suite-granularity shard plans are now byte-stable: XCTestEnumerator requests machine-parseable JSON written to a file (-test-enumeration-format json, -test-enumeration-style hierarchical, -test-enumeration-output-path) instead of scraping xcodebuild stdout, which could lose whole target entries on large bundles. ShardPlanService re-enumerates any module the bulk pass dropped and fails the shard plan loudly if a module still cannot be enumerated. Previously the same commit could produce plans of wildly different sizes across runs.