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

tests/borrow_test.nim

@@ -1,5 +1,5 @@
-import std/[unittest, os, strutils]
-import ../bootstrap/[lexer, parser, sema, types]
+import std/[unittest, strutils]
+import ../bootstrap/[lexer, parser, sema]
 
 proc checkSource(source: string): tuple[hasErrors: bool, diagnostics: seq[SemaDiagnostic]] =
   let lexRes = lexer.tokenize(source, "test.bux")