feat: logger_new y logger_with sobre log/slog (infra)

- logger_new (impuro): construye *Logger con handler JSON/text segun formato
- logger_with (puro): clona el Logger anadiendo campos contextuales via slog.With

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

functions/infra/logger_new.go

@@ -0,0 +1,54 @@
+package infra
+
+import (
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+)
+
+// LoggerNew crea un Logger con nivel, destino y formato configurables.
+// format debe ser "json" o "text". Si output es nil se usa os.Stderr.
+// Retorna error si el formato no es valido.
+func LoggerNew(level LogLevel, output io.Writer, format string) (*Logger, error) {
+	if output == nil {
+		output = os.Stderr
+	}
+
+	slogLevel := toSlogLevel(level)
+	opts := &slog.HandlerOptions{Level: slogLevel}
+
+	var handler slog.Handler
+	switch format {
+	case "json":
+		handler = slog.NewJSONHandler(output, opts)
+	case "text":
+		handler = slog.NewTextHandler(output, opts)
+	default:
+		return nil, fmt.Errorf("logger_new: formato invalido %q, usa \"json\" o \"text\"", format)
+	}
+
+	return &Logger{
+		Level:  level,
+		Output: output,
+		Format: format,
+		Fields: map[string]any{},
+		inner:  slog.New(handler),
+	}, nil
+}
+
+// toSlogLevel convierte LogLevel a slog.Level.
+func toSlogLevel(level LogLevel) slog.Level {
+	switch level {
+	case LogLevelDebug:
+		return slog.LevelDebug
+	case LogLevelInfo:
+		return slog.LevelInfo
+	case LogLevelWarn:
+		return slog.LevelWarn
+	case LogLevelError:
+		return slog.LevelError
+	default:
+		return slog.LevelInfo
+	}
+}