feat(teacher): stream Yuxi reasoning and answer previews

shared/teacher-reply.ts

@@ -224,6 +224,44 @@ export function isTeacherReplyEnvelope(raw: string): boolean {
   return withinBytes(raw, MAX_TEACHER_REPLY_BYTES) && candidates(raw.trim()).length > 0;
 }
 
+/** Preview received prose only; final validation still belongs to parseTeacherReply. */
+export function previewTeacherReply(raw: string): string {
+  if (!withinBytes(raw, MAX_TEACHER_REPLY_BYTES)) return '';
+  const text = raw.trimStart();
+  const found = candidates(text);
+  if (!found.length) {
+    // Wait for enough of an opening JSON/fence to identify the reply envelope.
+    if (/^\{/.test(text)) {
+      try { JSON.parse(text); } catch { return ''; }
+    }
+    if (/^(`{1,3}|~{1,3})([^\n]*)$/.test(text)
+      || /^(?:```|~~~)(?:json|makelore-teacher(?:-reply|-discussion)?)?\s*\n\s*\{?[^:]*$/i.test(text)) return '';
+    return text;
+  }
+  if (found.length !== 1) return '';
+  const candidate = found[0];
+  const scanned = fields(candidate.content);
+  if (scanned.duplicate || scanned.proseStart === undefined) return '';
+  const start = scanned.proseStart;
+  if (candidate.content[start] !== '"') return '';
+  let end = start + 1;
+  while (end < candidate.content.length) {
+    const char = candidate.content[end];
+    if (char === '"') break;
+    if (char === '\\') {
+      const size = candidate.content[end + 1] === 'u' ? 6 : 2;
+      if (end + size > candidate.content.length) break;
+      end += size;
+    } else end++;
+  }
+  try {
+    // Incomplete escapes remain buffered, never displayed as raw protocol text.
+    const prose: string = JSON.parse('"' + candidate.content.slice(start + 1, end) + '"');
+    return [text.slice(0, candidate.start).trim(), prose.replace(/[\uD800-\uDBFF]$/, '')]
+      .filter(Boolean).join('\n\n');
+  } catch { return ''; }
+}
+
 function projectReply(envelope: Record<string, unknown>): TeacherReply {
   const reply = own(envelope, 'reply') ? envelope.reply : envelope.intro;
   const suggestions = own(envelope, 'quickReplies') ? envelope.quickReplies : envelope.questions;