feat: funciones impuras HTTP — response, parse, logger, router, serve

Seis funciones de servidor HTTP con tests usando httptest:
- HTTPJSONResponse: escribe JSON con Content-Type y status code
- HTTPErrorResponse: escribe HTTPError como JSON estructurado
- HTTPParseBody: decode JSON con limite de bytes y campos estrictos
- HTTPLoggerMiddleware: loguea method/path/status/duration a io.Writer
- HTTPRouter: crea ServeMux con rutas Go 1.22+ (METHOD /path)
- HTTPServe: ListenAndServe con graceful shutdown por contexto

23 tests pasando, solo stdlib net/http.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

functions/infra/http_logger_middleware.go

@@ -0,0 +1,34 @@
+package infra
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+// responseWriter captura el status code escrito al ResponseWriter.
+type responseWriter struct {
+	http.ResponseWriter
+	status int
+}
+
+func (rw *responseWriter) WriteHeader(status int) {
+	rw.status = status
+	rw.ResponseWriter.WriteHeader(status)
+}
+
+// HTTPLoggerMiddleware retorna un Middleware que loguea metodo, path, status y duracion de cada request.
+// El formato de cada linea es: METHOD /path STATUS DURACIONms
+func HTTPLoggerMiddleware(logger io.Writer) Middleware {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			start := time.Now()
+			rw := &responseWriter{ResponseWriter: w, status: http.StatusOK}
+			next.ServeHTTP(rw, r)
+			duration := time.Since(start)
+			fmt.Fprintf(logger, "%s %s %d %dms\n",
+				r.Method, r.URL.Path, rw.status, duration.Milliseconds())
+		})
+	}
+}