terminal: store title set by escape sequences

Add a title field to Terminal, mirroring the existing pwd field.
The title is set via setTitle/getTitle and tracks the most recent
value written by OSC 0/2 sequences. The stream handler now persists
the title in terminal state in addition to forwarding it to the
surface. The field is cleared on full reset.

src/termio/stream_handler.zig

@@ -999,6 +999,12 @@ pub const StreamHandler = struct {
             return;
         }
 
+        // Set the title on the terminal state. We ignore any errors since
+        // we can continue to operate just fine without it.
+        self.terminal.setTitle(title) catch |err| {
+            log.warn("error setting title in terminal state: {e}", .{err});
+        };
+
         @memcpy(buf[0..title.len], title);
         buf[title.len] = 0;