feat(nexus): production modular HTTP/1.1 server + compiler fixes

- Rewrite apps/nexus with modular architecture:
  Config, Http, Errors, Parser, Router, Handlers, Server, Main
- Use algebraic enums for ParseResult/FileResult/HttpError
- Thread-pool server via Channel<ConnectionTask> and spawn
- Fix C backend type ordering for generic struct instances
  (Array_T, Iter_T) and algebraic enum struct payloads
- Collect Slice_T types from struct fields and enum payloads
- Fix match lowering for simple enums (direct value compare)
- Resolve match expression return type from first arm
- Infer element type for for-in over Array<UserStruct>
- Preserve generic type args in field access resolution
- Add fflush to PrintLine/Print for immediate server logs
- Add modern_features golden regression test
- Regenerate golden expected.c files

apps/nexus/src/Router.bux

@@ -0,0 +1,46 @@
+module Router {
+
+import Std::Array::{Array};
+import Std::String::{String_Eq};
+import Http::{HttpMethod, HttpRequest, HttpResponse, Http_NewResponse};
+import Handlers::{ServeStaticFile, HandleApiHealth, HandleApiInfo, HandleWebSocketUpgrade, NotFoundResponse};
+
+pub enum Handler {
+    StaticFile,
+    ApiHealth,
+    ApiInfo,
+    WsUpgrade,
+    NotFound,
+}
+
+pub struct Route {
+    method: HttpMethod;
+    path: String;
+    handler: Handler;
+}
+
+pub struct Router {
+    routes: Array<Route>;
+    notFound: Handler;
+}
+
+pub func Handler_Handle(h: Handler, req: HttpRequest) -> HttpResponse {
+    match h {
+        Handler::StaticFile => ServeStaticFile(req),
+        Handler::ApiHealth  => HandleApiHealth(),
+        Handler::ApiInfo    => HandleApiInfo(),
+        Handler::WsUpgrade  => HandleWebSocketUpgrade(req),
+        Handler::NotFound   => NotFoundResponse(),
+    }
+}
+
+pub func Router_Dispatch(r: Router, req: HttpRequest) -> HttpResponse {
+    for route in r.routes {
+        if route.method == req.method && String_Eq(route.path, req.path) {
+            return Handler_Handle(route.handler, req);
+        }
+    }
+    return Handler_Handle(r.notFound, req);
+}
+
+}