Add Target.targetInfoChanged and DOMSnapshot.captureSnapshot for DOM agents (#282)

CDP DOM-agent frameworks (browser-use and similar) could connect but saw a
blank page: navigation never refreshed their cached target url/title, and their
DOM build aborted with "Unknown domain: DOMSnapshot".

- Emit Target.targetInfoChanged after navigation (both CDP Page.navigate and
  JS-initiated) carrying the post-nav url/title. Strict clients refresh a target
  only on this event; it also fixes Puppeteer/Playwright page.url() tracking.
- Implement DOMSnapshot.captureSnapshot, built from the DOM with synthetic
  geometry (no paint engine) and plausible computed styles, keyed by the same
  backendNodeId as DOM.getDocument. Enough for tag/ARIA/accessibility element
  detection; it runs only when a client requests it, so the fetch path is
  unaffected. Coordinate clicking still requires a real layout engine.

crates/obscura-cdp/src/dispatch.rs

@@ -326,6 +326,7 @@ pub async fn dispatch(req: &CdpRequest, ctx: &mut CdpContext) -> CdpResponse {
         "Browser" => domains::browser::handle(method, &req.params).await,
         "Page" => domains::page::handle(method, &req.params, ctx, &req.session_id).await,
         "DOM" => domains::dom::handle(method, &req.params, ctx, &req.session_id).await,
+        "DOMSnapshot" => domains::domsnapshot::handle(method, &req.params, ctx, &req.session_id).await,
         "Runtime" => domains::runtime::handle(method, &req.params, ctx, &req.session_id).await,
         "Network" => domains::network::handle(method, &req.params, ctx, &req.session_id).await,
         "Fetch" => domains::fetch::handle(method, &req.params, ctx, &req.session_id).await,