[ 2024-12-02; Jonathan provides wording ]

I suggest that `from_stream` should only be found via ADL, not unqualified lookup. This is consistent with what we did for `make_error_code` and `make_error_condition`, and more recently for `submdspan_mapping`. I see no reason to treat `from_stream` differently. This implies that implementations might need a poison poll in `std::chrono` so that unqualified lookup stops as soon as those are found.