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

bootstrap/manifest.nim

@@ -90,7 +90,6 @@ proc parseInlineTable(s: string): OrderedTableRef[string, TomlValue] =
         if content[i] == '{': inc braceCount
         elif content[i] == '}': dec braceCount
         inc i
-      let val = content[valStart ..< i]
       result[key] = TomlValue(kind: tvkInlineTable, inlineVal: newOrderedTable[string, TomlValue]())
     else:
       while i < content.len and content[i] notin {',', ' ', '\t'}: