Enriches timeline events with per-assign byte size information.

Replaces the old Memory enricher. Tracks individual assign sizes plus total memory, enabling the assign_diff analyzer to detect large assigns that get re-diffed frequently.

Example Output

%{
  assign_sizes: %{current_user: 12400, products: 8200, filter: 120},
  total_memory: 20720,
  largest_assign: {:current_user, 12400}
}