ConsoleHandler found the file and line it prints by walking a fixed
number of stack frames up from itself, which only fit a record logged
through a slog.Logger and delivered through MultiplexHandler. It now
reads them from the record's PC, the call site slog stores, so a
ConsoleHandler used with slog.New and a record passed to Handle
directly print the right location too. A record whose PC is zero
prints ???:0 as before. The README drops its warning about the wrong
location and says how to set PC instead.
Model: opus-5-5
