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/Config.bux

@@ -0,0 +1,21 @@
+module Config {
+
+pub struct ServerConfig {
+    bindAddr: String;
+    port: int;
+    workerCount: int;
+    publicDir: String;
+    backlog: int;
+}
+
+pub const func DefaultConfig() -> ServerConfig {
+    return ServerConfig {
+        bindAddr: "0.0.0.0",
+        port: 8080,
+        workerCount: 4,
+        publicDir: "public",
+        backlog: 128,
+    };
+}
+
+}