fix(supertonic): decompose Cyrillic й/ё/ї into base letter + combining mark

The Supertonic unicode indexer maps most basic Cyrillic letters, but
й/Й, ё/Ё, and ї/Ї are absent (mapped to -1 in unicode_indexer.json),
causing "unicode indexer has no entry for codepoint N" errors for any
Russian or Ukrainian text containing these letters — both "ru" and
"uk" are listed as supported languages.

The indexer does contain entries for the combining breve (U+0306) and
combining diaeresis (U+0308), matching each letter's canonical NFD
decomposition (й = и + breve, ё = е + diaeresis, ї = і + diaeresis).
Decompose these codepoints the same way the tokenizer already handles
Hangul and Japanese kana dakuten/handakuten forms.

Author: Anc813

src/models/supertonic/tokenizer_text.cpp

@@ -139,13 +139,47 @@ bool append_japanese_kana_decomposition(uint32_t codepoint, std::vector<uint32_t
     return false;
 }
 
+bool append_cyrillic_decomposition(uint32_t codepoint, std::vector<uint32_t> & out) {
+    constexpr uint32_t kBreve = 0x0306;
+    constexpr uint32_t kDiaeresis = 0x0308;
+    static constexpr std::pair<uint32_t, uint32_t> kBreveLetters[] = {
+        {0x0439, 0x0438},  // й -> и
+        {0x0419, 0x0418},  // Й -> И
+    };
+    static constexpr std::pair<uint32_t, uint32_t> kDiaeresisLetters[] = {
+        {0x0451, 0x0435},  // ё -> е
+        {0x0401, 0x0415},  // Ё -> Е
+        {0x0457, 0x0456},  // ї -> і
+        {0x0407, 0x0406},  // Ї -> І
+    };
+
+    for (const auto & [composed, base] : kBreveLetters) {
+        if (codepoint == composed) {
+            out.push_back(base);
+            out.push_back(kBreve);
+            return true;
+        }
+    }
+    for (const auto & [composed, base] : kDiaeresisLetters) {
+        if (codepoint == composed) {
+            out.push_back(base);
+            out.push_back(kDiaeresis);
+            return true;
+        }
+    }
+    return false;
+}
+
 std::vector<uint32_t> decompose_known_text_codepoints(const std::vector<uint32_t> & codepoints) {
     std::vector<uint32_t> out;
     out.reserve(codepoints.size() * 2);
     for (const uint32_t codepoint : codepoints) {
         if (append_japanese_kana_decomposition(codepoint, out)) {
             continue;
         }
+        if (append_cyrillic_decomposition(codepoint, out)) {
+            continue;
+        }
         append_hangul_decomposition(codepoint, out);
     }
     return out;