Pin the last untested-but-correct paths surfaced by the coverage audit:
flow-style emission of double scalars, BatchReport.description across
multiple sources, the unkeyed superDecoder reading successive indexes,
the PreflightScanner scheme-colon exception that must not flag a missing
mapping space, and the tagged stream-validation throwing overload that
wraps failures in a document-indexed issue collection. Each assertion was
measured against actual behavior before being pinned.
