feat: 枚举 enum (P1 #7)

- lexer: TOK_ENUM, TOK_COLON_COLON
- ast: AST_ENUM_DECL, AST_ENUM_VARIANT + AST_PROGRAM enums数组
- parser: enum Name { A, B } + Enum::Variant语法
- sema: SYM_ENUM, 变体验证, enum→i64类型兼容
- codegen: TYPE_ENUM→i64, 变体→ConstInt(索引值)
- 新增集成测试: 17_enum.l

测试: 121 通过 (41+15+47+18)

src/lexer/lexer.c

@@ -63,6 +63,7 @@ static TokenKind check_keyword(const Token* tok) {
     KW("bool",   TOK_BOOL);   KW("str",    TOK_STR);
     KW("void",   TOK_VOID);
     KW("struct", TOK_STRUCT); KW("type", TOK_TYPE);
+    KW("enum", TOK_ENUM);
     KW("true",   TOK_TRUE);   KW("false",  TOK_FALSE);
     #undef KW
     return TOK_IDENT;
@@ -141,6 +142,7 @@ Token* lex(Arena* a, const char* source, const char* filename,
         else if (c == '{') { tokens[idx++] = make_token(&l, TOK_LBRACE, l.pos, 1); advance(&l); }
         else if (c == '}') { tokens[idx++] = make_token(&l, TOK_RBRACE, l.pos, 1); advance(&l); }
         else if (c == ',') { tokens[idx++] = make_token(&l, TOK_COMMA, l.pos, 1); advance(&l); }
+        else if (c == ':' && peek_next(&l) == ':') { tokens[idx++] = make_token(&l, TOK_COLON_COLON, l.pos, 2); advance(&l); advance(&l); }
         else if (c == ':') { tokens[idx++] = make_token(&l, TOK_COLON, l.pos, 1); advance(&l); }
         else if (c == ';') { tokens[idx++] = make_token(&l, TOK_SEMICOLON, l.pos, 1); advance(&l); }
         else {