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/lexer.nim

@@ -61,12 +61,6 @@ proc advance(lex: var Lexer): char =
     else:
       inc lex.col
 
-proc match(lex: var Lexer, expected: char): bool =
-  if lex.isAtEnd(): return false
-  if lex.peek() != expected: return false
-  discard lex.advance()
-  return true
-
 proc matchStr(lex: var Lexer, s: string): bool =
   for i, c in s:
     if lex.peek(i) != c:
@@ -81,9 +75,6 @@ proc currentLocation(lex: Lexer): SourceLocation =
 proc emitError(lex: var Lexer, loc: SourceLocation, message: string) =
   lex.diagnostics.add(LexerDiagnostic(severity: ldsError, loc: loc, message: message))
 
-proc emitWarning(lex: var Lexer, loc: SourceLocation, message: string) =
-  lex.diagnostics.add(LexerDiagnostic(severity: ldsWarning, loc: loc, message: message))
-
 proc makeToken(lex: Lexer, kind: TokenKind, startLoc: SourceLocation, startPos: int): Token =
   let text = lex.source[startPos ..< lex.pos]
   result = Token(kind: kind, text: text, loc: startLoc)
@@ -337,19 +328,6 @@ proc scanSymbol(lex: var Lexer, startLoc: SourceLocation): Token =
     else:
       return lex.makeToken(kind1, startLoc, startPos)
 
-  template check3(c2: char, kind2: TokenKind, c3: char, kind3: TokenKind, kind1: TokenKind) =
-    if lex.peek() == c2:
-      discard lex.advance()
-      if lex.peek() == c3:
-        discard lex.advance()
-        return lex.makeToken(kind3, startLoc, startPos)
-      return lex.makeToken(kind2, startLoc, startPos)
-    else:
-      return lex.makeToken(kind1, startLoc, startPos)
-
-  template checkEq(c2: char, kind2: TokenKind, kind1: TokenKind) =
-    check2(c2, kind2, kind1)
-
   case c1
   of '(': return lex.makeToken(tkLParen, startLoc, startPos)
   of ')': return lex.makeToken(tkRParen, startLoc, startPos)