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

TODO.md

@@ -24,6 +24,10 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
 
 # Completed Steps
 
+* 2026-10-06: the console handler takes the file and line it prints from
+    the record's `PC` instead of counting stack frames, so they name the
+    call site whether the record came through a `slog.Logger` or straight
+    to `Handle`
 * 2026-10-06: the tests run under the race detector, in Docker:
     `script/test` builds the `test` stage of the `Dockerfile`, which runs
     `go test -race`, and a new final stage makes a plain `docker build`