cli: list fetch()/XHR resources in --dump assets

--dump assets only walked static DOM attributes (script[src], img[src], ...),
so resources a page pulled in with fetch() or XHR never showed up (issue #301).

Record each absolute URL requested through op_fetch_url in ObscuraState,
expose it via the runtime and Page, and append those URLs to --dump assets as
type "fetch", deduped against URLs already listed from the DOM.

crates/obscura-browser/src/page.rs

@@ -1211,6 +1211,12 @@ impl Page {
         self.dom.as_ref().map(f)
     }
 
+    /// Absolute URLs the page pulled in via fetch()/XHR (issue #301). Empty
+    /// when the page has no live JS runtime.
+    pub fn fetched_urls(&self) -> Vec<String> {
+        self.js.as_ref().map(|js| js.fetched_urls()).unwrap_or_default()
+    }
+
     pub fn dom(&self) -> Option<&DomTree> {
         self.dom.as_ref()
     }