Merge v3-t39: Rust builder behind host.sausage/cargo and the world cargo CLI (T39, P4)

# Conflicts:
#	flake.nix
#	host/src/process/mod.rs
#	host/tests/hook_git.rs
#	host/tests/hook_guest.rs
#	host/tests/process.rs
#	userland/Cargo.toml

host/src/process/http.rs

@@ -45,9 +45,14 @@ impl WasiHttpHooks for Hooks {
                 .is_some_and(|h| h.eq_ignore_ascii_case("host.sausage"))
             {
                 let bytes = body.collect().await?.to_bytes();
-                return Ok(response(
-                    crate::api::handle(&router, caller, Request::from_parts(parts, bytes)).await,
-                ));
+                let request = Request::from_parts(parts, bytes);
+                if request.method() == http::Method::POST && request.uri().path() == "/cargo" {
+                    return Ok((
+                        crate::api::stream(&router, request),
+                        Box::new(async { Ok(()) }) as Io,
+                    ));
+                }
+                return Ok(response(crate::api::handle(&router, caller, request).await));
             }
             if !matches!(parts.uri.scheme_str(), Some("http" | "https")) {
                 while let Some(frame) = body.frame().await {

host/src/process/mod.rs

@@ -193,6 +193,10 @@ impl Runtime {
             )?),
         };
         router.attach_journal(world.root.journal());
+        let _ = router.builder.set(Arc::new(crate::builder::Builder::new(
+            world.root.clone(),
+            &world.host_dir,
+        )?));
         let network = Arc::new(net::NetPolicy::open(&world.host_dir)?);
         let engine = compiler.engine.clone();
         let alive = Arc::new(AtomicBool::new(true));