Let a handler flush or set a write deadline through the access log and metrics (closes #191)

The access log's response writer and the metrics middleware's writer hid the writer beneath them, so a handler's flush, hijack or write deadline set through http.ResponseController failed with "not supported" behind them. The access log's writer now has Unwrap. The metrics middleware calls the library's public Measure with a writer of our own that has Unwrap, in place of std.Handler's writer, so the middleware order and what metrics record are unchanged. A test over a real connection sets a write deadline and flushes, metrics on and off, on a global route and in an admin page group, and fails without either Unwrap.

Model: opus-5-5

internal/middleware/middleware.go

@@ -233,6 +233,13 @@ func (lrw *loggingResponseWriter) WriteHeader(code int) {
 	lrw.ResponseWriter.WriteHeader(code)
 }
 
+// Unwrap lets http.ResponseController reach the writer underneath, so
+// a handler can still flush or set a write deadline through the access
+// log.
+func (lrw *loggingResponseWriter) Unwrap() http.ResponseWriter {
+	return lrw.ResponseWriter
+}
+
 // concreteLogURL renders the request's own URL for the access log
 // branches that keep it, with the query string replaced by a fixed
 // marker.