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_router.go

@@ -0,0 +1,17 @@
+package infra
+
+import (
+	"fmt"
+	"net/http"
+)
+
+// HTTPRouter crea un http.ServeMux y registra las rutas dadas.
+// Usa la sintaxis de Go 1.22+: "METHOD /path" como patron del mux.
+func HTTPRouter(routes []Route) *http.ServeMux {
+	mux := http.NewServeMux()
+	for _, route := range routes {
+		pattern := fmt.Sprintf("%s %s", route.Method, route.Path)
+		mux.Handle(pattern, route.Handler)
+	}
+	return mux
+}