feat: report optional render performance metrics

README.md

@@ -24,6 +24,10 @@ cargo run -p ferese -- foot
 
 Set `RUST_LOG=ferese=debug` for detailed compositor logging.
 
+Set `FERESE_TRACE_PERFORMANCE=1` to emit five-second per-output summaries under
+the `ferese::render` tracing target. Summaries include rendered frames, damaged
+pixels, average and longest render time, and DRM missed-deadline totals.
+
 Ferese reads `$XDG_CONFIG_HOME/ferese/config.toml`, or
 `~/.config/ferese/config.toml` when `XDG_CONFIG_HOME` is unset. New scrolling
 columns use half the viewport by default. To make every newly opened column use