ux: Pane.flushLayoutSync — synchronous layout pass for inline measurement

Subclasses that need a child's real measured size at gesture-time (not
the next frame) can set a per-class measure-request field and call this
to make `performLayout` run synchronously, observe the field, and
populate sizes. No extra frame scheduled.

Banlu's chat-stream uses this for scroll-to-reply on far-back history
that hasn't been laid out yet — without it, the path is either iterative
single-viewport jumps (waits for layout per jump) or minHeight estimates
that misalign the landing position.

lib/src/anim/pane.dart

@@ -90,6 +90,19 @@ abstract class Pane with Emitter {
     return child;
   }
 
+  /// Forces a synchronous layout pass on this Pane. Callers that need a
+  /// slot's real measured size *now* (e.g. scroll-to-message in a chat
+  /// that hasn't laid out far-back history yet) set up a per-subclass
+  /// "measure request" field, then invoke this — the next [performLayout]
+  /// observes the field and measures the requested slots inline. No
+  /// extra frame is scheduled.
+  void flushLayoutSync() {
+    final owner = renderer?.owner;
+    if (owner == null) return;
+    renderer!.markNeedsLayout();
+    owner.flushLayout();
+  }
+
   void invokeLayoutRemove(Object? slot) {
     if (element == null) return;
     renderer?.invokeLayoutCallback((constraints) {