Event::notify takes std::uint64_t but the header never included <cstdint>,
relying on it arriving through another include. clang-tidy's include-cleaner
flags it, which fails CI on any branch where this file lands in the
changed-file set.
Fixed here, on the branch that introduced the std::uint64_t parameter, rather
than only downstream where it happened to surface.
2f8d8eabc7