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/parser.nim

@@ -62,12 +62,6 @@ proc checkAny(p: Parser, kinds: openArray[TokenKind]): bool =
     if p.peek() == k: return true
   return false
 
-proc match(p: var Parser, kind: TokenKind): bool =
-  if p.check(kind):
-    discard p.advance()
-    return true
-  return false
-
 proc isTypeArgListAhead(p: Parser): bool =
   ## Lookahead to determine if '<' starts a type argument list.
   ## Returns true if we can find a matching '>' before EOF, '{', or ';'.
@@ -788,7 +782,6 @@ proc parseShift(p: var Parser): Expr =
   return left
 
 proc parseCast(p: var Parser): Expr =
-  let loc = p.currentLoc
   var left = p.parseShift()
   # 'as' and 'is' are handled in postfix for chaining
   return left
@@ -1171,7 +1164,6 @@ proc parseParamList(p: var Parser, allowVariadic: bool = false): seq[Param] =
     if p.check(tkRParen) or p.isAtEnd:
       break
     let loc = p.currentLoc
-    var isVar = false
     if allowVariadic and p.check(tkDotDotDot):
       discard p.advance()
       let nameTok = p.at