Suite-granularity shard enumeration is reliable

Suite-granularity shard enumeration now reads xcodebuild’s machine-parseable JSON enumeration file (-test-enumeration-format json -test-enumeration-style hierarchical -test-enumeration-output-path) instead of scraping streamed stdout, which was truncating and interleaving under load. The plan is anchored on the deterministic .xctestrun module set with per-target re-enumeration for modules the bulk pass dropped; modules that remain unenumerable throw modulesFailedToEnumerate rather than being silently dropped.