rproxy, the proxy in front of each app, asks servewaf and fails open (closes #171)

`smallwebwaf rproxy` reads the first 64 KiB of each body, asks servewaf over
HTTP/1.1, and passes what servewaf allows to the app within its limits. Any
other answer goes to the visitor as servewaf gave it. SWRP_DEFAULT_DISPOSITION
decides only when servewaf cannot be connected to, does not answer at all, or
does not answer within SWRP_TIMEOUT_MS. A method SPEC.md does not list gets 405.

Duplication: internal/rproxy copies passing-on code from internal/proxy until
#172 removes the originals.
Rule suppressed: gosec G704 and G705; gosec at
internal/smallwebwaf/rproxy_test.go:431; tagliatelle at
internal/requestlog/requestlog.go:203; paralleltest on one test.
Judgement call: an answer over 1 MiB, or with a status below 200, gets 502.
Judgement call: every target not starting with one / gets 400.

Model: opus-5-5

internal/requestlog/requestlog_test.go

@@ -67,6 +67,28 @@ func TestWriteWritesOneJSONLineMarkedRequest(t *testing.T) {
 	}
 }
 
+func TestWriteRProxyLineWritesOneJSONLineWithOnlyItsFields(t *testing.T) {
+	t.Parallel()
+
+	var out bytes.Buffer
+
+	err := requestlog.WriteRProxyLine(&out, &requestlog.RProxyLine{
+		RequestID:   "an-id",
+		Disposition: "allow",
+	})
+	if err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	const want = `{"type":"request","time":"","request_id":"an-id","client_ip":"",` +
+		`"host":"","method":"","path":"","query":"","status":0,"request_bytes":0,` +
+		`"response_bytes":0,"duration_request":0,"duration_response":0,` +
+		`"disposition":"allow"}` + "\n"
+	if out.String() != want {
+		t.Errorf("wrote %s, want %s", out.String(), want)
+	}
+}
+
 func TestProcessLinesAreMarkedProcessAndGiveTheInstance(t *testing.T) {
 	t.Parallel()