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

Makefile

@@ -78,7 +78,7 @@ selfhost: build
 
 .PHONY: test-golden
 
-GOLDEN_TESTS := hello fibonacci structs generics algebraic_enums enums methods strings
+GOLDEN_TESTS := hello fibonacci structs generics algebraic_enums enums methods strings modern_features
 
 test-golden: build
 	@echo "=== Golden tests ==="