test: add native client soak harness

README.md

@@ -28,6 +28,9 @@ 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.
 
+The native-client soak runner and its nested/DRM procedure are documented in
+[docs/native-soak-testing.md](docs/native-soak-testing.md).
+
 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