fix: use real UTF-8 for jstring -> native conversion

JNI's GetStringUTFChars emits MUTF-8, which encodes codepoints above
U+FFFF as a surrogate-pair of two 3-byte sequences. ImGui's UTF-8 decoder
rejects those, so SMP emoji (U+1F000+) never reach the glyph atlas even
when the glyph is registered.

Add Jni::ToTrueUtf8 / Jni::FreeTrueUtf8 helpers in jni_common that decode
via GetStringChars (UTF-16) and emit proper 1/2/3/4-byte UTF-8, then
route every generator- and manually-written JNI call site through them
(1921 occurrences across the generated tree and 3 hand-written files,
plus the jni_content.kt generator template so future regenerations stay
consistent).

buildSrc/src/main/kotlin/tool/generator/api/jni_content.kt

@@ -350,8 +350,8 @@ private fun convertManualJni(mOrig: CtMethod<*>, method: CtMethod<*>): CtMethod<
         }
 
         if (p.isType("String")) {
-            getLines += "auto ${p.simpleName} = obj_${p.simpleName} == NULL ? NULL : (char*)env->GetStringUTFChars(obj_${p.simpleName}, JNI_FALSE)"
-            releaseLines += "if (${p.simpleName} != NULL) env->ReleaseStringUTFChars(obj_${p.simpleName}, ${p.simpleName})"
+            getLines += "auto ${p.simpleName} = Jni::ToTrueUtf8(env, obj_${p.simpleName})"
+            releaseLines += "Jni::FreeTrueUtf8(${p.simpleName})"
             isConverted = true
         }
 
@@ -361,14 +361,12 @@ private fun convertManualJni(mOrig: CtMethod<*>, method: CtMethod<*>): CtMethod<
                 const char* ${p.simpleName}[$countParam];
                 for (int i = 0; i < ${p.simpleName}$PARAM_ARR_LEN_POSTFIX; i++) {
                     const jstring str = (jstring)env->GetObjectArrayElement(obj_${p.simpleName}, i);
-                    auto rawStr = (char*)env->GetStringUTFChars(str, JNI_FALSE);
-                    ${p.simpleName}[i] = rawStr;
+                    ${p.simpleName}[i] = Jni::ToTrueUtf8(env, str);
                 }
             """.trimIndent()
             releaseLines += """
                 for (int i = 0; i < $countParam; i++) {
-                    const jstring str = (jstring)env->GetObjectArrayElement(obj_${p.simpleName}, i);
-                    env->ReleaseStringUTFChars(str, ${p.simpleName}[i]);
+                    Jni::FreeTrueUtf8((char*)${p.simpleName}[i]);
                 }
             """.trimIndent()
             isConverted = true