feat(cdp): stream large response bodies via takeResponseBodyAsStream + IO (#381)

Network.getResponseBody returns the whole body in one base64 blob and the
cache drops anything over 2 MiB, so large downloads (images, PDFs,
archives) either failed or forced the long-running server to hold the
full body in memory (issue #360).

Add Fetch.takeResponseBodyAsStream + IO.read + IO.close. The body is moved
out of the page cache into a stream handle and delivered in bounded chunks
via IO.read, then freed on IO.close, so it is held once and released after
the download. Binary bodies are byte-exact (stored base64). To stream a
large body, raise OBSCURA_NETWORK_BODY_BUFFER_BYTES so it is cached.

This is entirely opt-in: nothing runs unless a client calls the new
methods, so the default fetch/render/scrape path is unchanged (verified:
obstacle course 33/33, obscura-vs-Chrome head-to-head within noise of the
pre-change baseline).

Fixes #360.

crates/obscura-cdp/src/dispatch.rs

@@ -45,6 +45,11 @@ pub struct CdpContext {
     pub next_isolated_context_id: i64,
     pub fetch_intercept: FetchInterceptState,
     pub intercept_tx: Option<tokio::sync::mpsc::UnboundedSender<InterceptedRequest>>,
+    // Open IO streams for Fetch.takeResponseBodyAsStream: handle -> (bytes, cursor).
+    // Each holds a response body taken out of the page cache so a large download
+    // is streamed chunk-by-chunk via IO.read and freed on IO.close (issue #360).
+    pub io_streams: HashMap<String, (Vec<u8>, usize)>,
+    pub io_stream_counter: u64,
 }
 
 impl CdpContext {
@@ -141,6 +146,8 @@ impl CdpContext {
             isolated_worlds: Vec::new(),
             valid_context_ids,
             next_isolated_context_id: 100,
+            io_streams: HashMap::new(),
+            io_stream_counter: 0,
         }
     }
 
@@ -334,6 +341,7 @@ pub async fn dispatch(req: &CdpRequest, ctx: &mut CdpContext) -> CdpResponse {
         "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,
+        "IO" => domains::io::handle(method, &req.params, ctx).await,
         "Input" => domains::input::handle(method, &req.params, ctx, &req.session_id).await,
         "Storage" => domains::storage::handle(method, &req.params, ctx, &req.session_id).await,
         "LP" => domains::lp::handle(method, &req.params, ctx, &req.session_id).await,