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

console_handler.go

@@ -12,10 +12,6 @@ import (
 	"github.com/fatih/color"
 )
 
-// callerSkipFrames is the number of stack frames between runtime.Caller
-// and the slog call site that produced the record.
-const callerSkipFrames = 4
-
 // ConsoleHandler writes human-readable, colored log lines to stdout.
 type ConsoleHandler struct {
 	// out is where records are written. Nil means os.Stdout, looked up on
@@ -53,11 +49,13 @@ func (c *ConsoleHandler) Handle(
 		colorFunc = color.New(color.FgWhite).SprintfFunc()
 	}
 
-	// Get the caller information
-	_, file, line, ok := runtime.Caller(callerSkipFrames)
-	if !ok {
-		file = "???"
-		line = 0
+	// The file and line come from the record's PC, the call site; a record
+	// without one prints a placeholder.
+	file, line := "???", 0
+
+	if record.PC != 0 {
+		frame, _ := runtime.CallersFrames([]uintptr{record.PC}).Next()
+		file, line = frame.File, frame.Line
 	}
 
 	out := c.out