chat: double-buffered sync with content-length diffing

Store content lengths of rendered entries. On each tick:
- Generation changed → full pane reset
- Entries removed → pop from tail
- Last entry content length changed → pop and re-render (streaming)
- New entries → route and push

PaneState gains pop_line() for removing the last rendered entry.
This handles streaming (last entry growing), compaction (generation
bump), and normal appends.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/user/mod.rs

@@ -190,6 +190,11 @@ impl PaneState {
         self.evict();
     }
 
+    pub(crate) fn pop_line(&mut self) {
+        self.lines.pop();
+        self.markers.pop();
+    }
+
     fn scroll_up(&mut self, n: u16) {
         self.scroll = self.scroll.saturating_sub(n);
         self.pinned = true;