api: add request/response interception API to Page (#306)

Expose the interception infrastructure that already existed internally but was
private, so embedders can intercept, observe, mock, or block JS fetch()/XHR
without forking or unsafe casts.

On obscura::Page:
- add_preload_script: inject JS before any page <script> (CDP
  Page.addScriptToEvaluateOnNewDocument). The field already ran the scripts;
  this only adds the method.
- enable_interception: returns a receiver for every JS fetch()/XHR request;
  resolve each through its resolver with InterceptResolution::{Continue,
  Fulfill, Fail} to pass, mock, or block (CDP Fetch.enable equivalent).
- on_request / on_response: passive callbacks. on_response is the main path for
  capturing API response payloads from SPAs.

on_request/on_response previously fired only for navigation; op_fetch_url now
invokes them for JS fetch()/XHR too (both reqwest and stealth paths), gated on
a non-empty callback vector so the common no-callback case pays nothing.

Re-export InterceptedRequest, InterceptResolution, RequestInfo, ResourceType,
RequestCallback, ResponseCallback, Response from the obscura crate.

Integration test tests/interception.rs exercises all three against a local echo
server.

crates/obscura-net/src/lib.rs

@@ -8,8 +8,8 @@ pub mod blocklist;
 pub mod wreq_client;
 
 pub use client::{
-    env_allows_private_network, is_forbidden_ip, ObscuraHttpClient, ObscuraNetError, RequestInfo,
-    ResourceType, Response, SsrfGuardResolver,
+    env_allows_private_network, is_forbidden_ip, ObscuraHttpClient, ObscuraNetError, RequestCallback,
+    RequestInfo, ResourceType, Response, ResponseCallback, SsrfGuardResolver,
 };
 pub use cookies::{CookieInfo, CookieJar};
 pub use encoding::{