H-391: preserve unknown cache state

Co-authored-by: multica-agent <github@multica.ai>

test/m0-spike.test.mjs

@@ -32,6 +32,15 @@ test("export policy redacts credentials and bounds bodies", () => {
   assert(!shouldRetryStartup(undefined));
 });
 
+test("cache state stays unknown without a preceding request event", () => {
+  const collector = eventCollector("session-a", "target-a");
+  collector.listener("Network.responseReceived", {
+    requestId: "orphan-response",
+    response: { url: "https://example.test/favicon.ico", status: 200, protocol: "h2", mimeType: "image/x-icon", headers: {} },
+  });
+  assert.deepEqual(collector.records[0].response.cache, { state: "unknown" });
+});
+
 test("WebSocket limits aggregate drops without payloads", () => {
   const collector = eventCollector("session-a", "target-a", {
     maxWebSocketFrameBytes: 4,