Take the console location from the record (closes #36)

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

README.md

@@ -126,8 +126,9 @@ error away. That is how `log/slog` works, and simplelog cannot change it.
 To find out whether a record was delivered, build a `slog.Record` and pass
 it to the handler yourself, for example
 `slog.Default().Handler().Handle(ctx, record)`, then check the error it
-returns. A record passed to `Handle` directly gets the wrong file and
-line in console output.
+returns. Console output takes the file and line from the record's `PC`,
+which you set with `runtime.Callers` as the `log/slog` documentation
+shows. A record whose `PC` is zero prints `???:0` instead.
 
 ## Entrypoints