feat(net): make passive request/response callbacks detachable (#416)

on_request/on_response stored callbacks in append-only Vecs with no way
to remove one, so a crawler that installed a response-body capture hook
for one phase could never detach it: it fired for the shared client's
whole lifetime, re-registering across navigations piled up duplicates,
and (because the client is shared per context) it saw sibling pages'
requests too.

Tag each callback with a stable id. on_request/on_response now return
the id and off_request/off_response (remove_on_request/remove_on_response
on the client) detach a specific one. Storage is Vec<(u64, Callback)>;
all fire sites destructure the tuple. Closes #408.

crates/obscura-js/src/ops.rs

@@ -792,7 +792,7 @@ async fn op_fetch_url(
                     headers: custom_headers.clone(),
                     resource_type: ResourceType::Fetch,
                 };
-                for cb in cbs.iter() {
+                for (_, cb) in cbs.iter() {
                     cb(&info);
                 }
             }
@@ -1038,7 +1038,7 @@ async fn op_fetch_url(
                 headers: resp_headers.clone(),
                 resource_type: ResourceType::Fetch,
             };
-            for cb in cbs.iter() {
+            for (_, cb) in cbs.iter() {
                 cb(&info, &resp);
             }
         }
@@ -1231,7 +1231,7 @@ async fn stealth_fetch_all(
                 headers: resp_headers.clone(),
                 resource_type: ResourceType::Fetch,
             };
-            for cb in cbs.iter() {
+            for (_, cb) in cbs.iter() {
                 cb(&info, &resp);
             }
         }