Add a `size` callback to the stream_terminal Effects struct that returns a size_report.Size geometry snapshot for XTWINOPS size queries (CSI 14/16/18 t). The handler owns all protocol encoding using the existing size_report.encode, keeping VT knowledge out of effect consumers. This follows the same pattern as the xtversion effect: the callback supplies data, the handler formats the reply and calls write_pty. CSI 21 t (title report) is handled internally from terminal state since the title is already available via terminal.getTitle() and does not require an external callback.