Suite-granularity shard enumeration is deterministic and complete

XCTestEnumerator now reads machine-parseable JSON written to a file via -test-enumeration-format json -test-enumeration-style hierarchical -test-enumeration-output-path instead of streaming human-readable text from stdout. Suite-granularity plans previously lost whole target entries to stream truncation, hiding dozens of targets on the same commit. A missing or malformed output file now fails loudly rather than returning a partially-enumerated set.