The previous span.setter accepted Union[Span, StreamedSpan] and, when given a StreamedSpan segment, populated self._transaction and self._transaction_info from the span's name and 'sentry.span.source' attribute. The new span.setter only handles Transaction and silently ignores StreamedSpan inputs at runtime. Although the type annotation is now Optional[Span], existing callers (integrations, user code, third-party code) that still assign a StreamedSpan via scope.span = streamed_span will no longer have transaction metadata populated, causing events to be missing transaction names/sources and breaking segment linkage in streaming mode. The PR's stated goal is backwards compatibility, but this is only type-level — runtime behavior regressed.