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-browser/src/lib.rs

@@ -7,3 +7,6 @@ pub use page::{NetworkEvent, Page, PageError};
 pub use context::BrowserContext;
 pub use lifecycle::{LifecycleState, WaitUntil};
 pub use obscura_js::HTML_TO_MARKDOWN_JS;
+// Re-exported so the embeddable `obscura` crate (which depends on obscura-browser,
+// not obscura-js) can surface the interception channel types.
+pub use obscura_js::ops::{InterceptResolution, InterceptedRequest};