Use CMU font only for Russian text

We want custom CMU Serif Roman to be used only for specific Russian text
and the main font to be default.

data/text.moi

@@ -340,7 +340,15 @@ Or Cyrillic (see table \tex_ref {tbl:model_cyrillic}).
 \table
     {tbl:model_cyrillic}
     {Modeling Cyrillic symbols}
-    {{Existing symbol} {Г} {Ж} {И} {П} {Ш} {Э}}
+    {
+        {Existing symbol}
+        {\ru {Г}}
+        {\ru {Ж}}
+        {\ru {И}}
+        {\ru {П}}
+        {\ru {Ш}}
+        {\ru {Э}}
+    }
     {
         {Proposed system}
         {\symbol {vl ht}}
@@ -420,7 +428,7 @@ recognizable.
 \table
     {tbl:model_other}
     {Modeling other symbols}
-    {{Existing symbol} {A} {M} {Y} {У} {Ф} {Ч}}
+    {{Existing symbol} {A} {M} {Y} {\ru {У}} {\ru {Ф}} {\ru {Ч}}}
     {
         {Proposed system}
         {\symbol {vl hto hc vr}}

python/moire_converter.py

@@ -26,6 +26,10 @@ def ko(self, arg) -> str:
         """Korean text symbol."""
         raise NotImplementedError()
 
+    def ru(self, arg) -> str:
+        """Russian text symbol."""
+        raise NotImplementedError()
+
     def symbol(self, arg) -> str:
         raise NotImplementedError()
 
@@ -41,9 +45,9 @@ def body(self, arg) -> str:
             \\usepackage{tikz}
             \\usepackage{fontspec}
             \\usepackage[left=1.25in,right=1.25in,top=1in,bottom=1in]{geometry}
-            \\setmainfont[Ligatures=TeX]{CMU Serif Roman}
             \\newfontface\\doulos{Doulos SIL}
             \\newfontface\\ko{Noto Sans KR Regular}
+            \\newfontface\\ru{CMU Serif Roman}
             \\def\\arraystretch{1.3}
             \\usetikzlibrary{positioning}
             \\begin{document}
@@ -129,6 +133,9 @@ def ipa(self, arg) -> str:
     def ko(self, arg) -> str:
         return f"{{\\ko{{{self.parse(arg[0])}}}}}"
 
+    def ru(self, arg) -> str:
+        return f"{{\\ru{{{self.parse(arg[0])}}}}}"
+
     def tikz_symbol(self, arg) -> str:
         proc: subprocess.Popen = subprocess.Popen(
             [SYMBOL_GENERATOR_EXECUTABLE, "symbol"]