test: add native client soak harness

docs/native-soak-testing.md

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+# Native-client soak testing
+
+The M6 soak runner repeatedly creates and destroys native Wayland clients while
+exercising focus, resize, column width, fullscreen, floating, and workspace
+paths. Every tenth iteration also terminates a client abruptly. The runner
+checks that the compositor and authenticated IPC socket remain responsive and
+records compositor RSS growth.
+
+Build Ferese and `feresectl`, start the compositor, and run:
+
+```bash
+FERESE_TRACE_PERFORMANCE=1 cargo run -p ferese -- foot
+```
+
+From a terminal connected to Ferese, or from the host with the Ferese socket
+selected, run:
+
+```bash
+WAYLAND_DISPLAY=wayland-1 \
+FERESE_PID="$(pgrep -n -x ferese)" \
+./scripts/soak-native.sh
+```
+
+The default duration is 24 hours. A shorter validation run is:
+
+```bash
+WAYLAND_DISPLAY=wayland-1 \
+FERESE_PID="$(pgrep -n -x ferese)" \
+FERESE_SOAK_SECONDS=300 \
+./scripts/soak-native.sh
+```
+
+Environment controls:
+
+- `FERESE_SOAK_SECONDS`: run duration, default `86400`;
+- `FERESE_SOAK_DELAY`: delay between operations, default `0.15` seconds;
+- `FERESE_SOAK_CLIENT`: native client executable, default `foot`;
+- `FERESECTL`: path to `feresectl`, default `target/debug/feresectl`;
+- `FERESE_PID`: compositor PID; automatic lookup is only a fallback;
+- `FERESE_SOAK_LOG`: result log, default `/tmp/ferese-soak-<pid>.log`.
+
+Run the same workload once with the nested backend and once from a direct DRM
+session. A passing run requires the compositor to remain alive, IPC queries to
+succeed, and RSS samples to stabilize rather than grow without bound. Preserve
+the soak log and the `ferese::render` summaries with the release artifacts.