fix: safety valve only flushes partial openers, not real tags

Long cleanup tags (e.g. collapse summaries) can span 30+ SSE deltas.
The safety valve was flushing after 6 deltas regardless, dumping
incomplete tags into the output. Now only flushes when buffering a
partial opener (<m, <y) that never resolved — never when inside a
confirmed tag.

src/mnemosyne/gateway.py

@@ -220,9 +220,16 @@ class SSECleanupFilter:
         has_close = bool(_TAG_CLOSE_RE.search(self._buf))
         partial = _has_partial_tag(self._buf)
 
-        # Safety valve: if we've buffered too many deltas without resolving
-        # a tag, this is just prose containing '<' — flush everything.
-        if self._buffered_deltas > self._MAX_BUFFERED_DELTAS and not has_close:
+        # Safety valve: if we've buffered many deltas waiting for a *partial*
+        # opener to resolve (e.g. "<m" that turned out to be prose "<model>"),
+        # flush everything.  But NEVER flush when _inside_tag is True — that
+        # means we matched a real opening tag and are waiting for the close.
+        # Real tags (e.g. long collapse summaries) can span 30+ deltas.
+        if (
+            self._buffered_deltas > self._MAX_BUFFERED_DELTAS
+            and not self._inside_tag
+            and not has_close
+        ):
             result = self._buf
             self._buf = ""
             self._inside_tag = False