Re-add labwc stress harness scripts

nested-labwc.sh runs a command inside a nested labwc (wlroots-family host)
so the clipboard bridge exercises the data-control backend; stress-clipboard.sh
loops the clipboard smoke tests, snapshotting logs into /tmp/stress-fail/ on
any failure.  Recovered verbatim after the history rewrite dropped scripts/.

Usage:
  scripts/nested-labwc.sh scripts/stress-clipboard.sh [iterations]

scripts/nested-labwc.sh

@@ -0,0 +1,72 @@
+#!/bin/sh
+# Throwaway dev wrapper: run a command inside a nested labwc (wlroots-family
+# host) so the clipboard bridge uses the wlr-data-control backend.
+# Usage: nested-labwc.sh [--x11] <command...>
+# Default command: tabbed-wlroots with two wltest tabs.
+set -e
+
+REPO="${TABBED_REPO:-/home/ahamilton/code/tabbed-wayland}"
+RUNTIME="${XDG_RUNTIME_DIR:-/run/user/$(id -u)}"
+export WAYLAND_DISPLAY="${WAYLAND_DISPLAY:-wayland-0}"
+export WLR_WL_OUTPUTS="${WLR_WL_OUTPUTS:-1}"
+
+MODE=wayland
+if [ "$1" = "--x11" ]; then
+    MODE=x11
+    shift
+fi
+
+if [ "$MODE" = x11 ]; then
+    export WLR_BACKENDS=x11
+    export DISPLAY="${DISPLAY:-:0}"
+    echo "[nested-labwc] x11 backend on $DISPLAY"
+else
+    export WLR_BACKENDS=wayland
+    echo "[nested-labwc] wayland backend under $WAYLAND_DISPLAY"
+fi
+
+before="$(ls "$RUNTIME"/wayland-* 2>/dev/null || true)"
+
+labwc -t "labwc-dev" >/tmp/labwc.log 2>&1 &
+LABWC_PID=$!
+trap 'kill "$LABWC_PID" 2>/dev/null || true' EXIT INT TERM
+
+# Discover labwc's socket: the wayland-N that wasn't there before. Only match
+# plain wayland-<n> sockets (labwc's name); portal/other compositors create
+# wayland-proxy-* sockets at arbitrary times and must be ignored.
+SOCK=""
+for i in $(seq 1 150); do
+    for s in "$RUNTIME"/wayland-*; do
+        [ -e "$s" ] || continue
+        b="${s##*/}"
+        case "$b" in *.lock|*proxy*) continue ;;
+        wayland-[0-9]*) case "$before" in *"$s"*) ;; *) SOCK="$b"; break 2 ;; esac ;;
+    esac
+    done
+    if ! kill -0 "$LABWC_PID" 2>/dev/null; then
+        echo "[nested-labwc] labwc exited early; tail:" >&2
+        tail -20 /tmp/labwc.log >&2
+        exit 1
+    fi
+    sleep 0.1
+done
+
+if [ -z "$SOCK" ]; then
+    echo "[nested-labwc] no new wayland socket appeared" >&2
+    tail -20 /tmp/labwc.log >&2
+    exit 1
+fi
+
+echo "[nested-labwc] nested labwc socket: $SOCK"
+export WAYLAND_DISPLAY="$SOCK"
+
+if [ "$#" -eq 0 ]; then
+    set -- "$REPO/build/tabbed-wlroots" \
+        "$REPO/build/wltest '#ff0000'" \
+        "$REPO/build/wltest '#0000ff'"
+fi
+
+"$@"
+rc=$?
+kill "$LABWC_PID" 2>/dev/null || true
+exit "$rc"

scripts/stress-clipboard.sh

@@ -0,0 +1,39 @@
+#!/usr/bin/env bash
+# Stress-run the clipboard smoke tests against a wlroots-family host.
+# Meant to be run inside scripts/nested-labwc.sh so the data-control
+# backend is exercised:
+#   scripts/nested-labwc.sh scripts/stress-clipboard.sh [iterations] [pytest-args...]
+# Prints PASS/FAIL per iteration and snapshots logs on any failure into
+# /tmp/stress-fail/iter-<n>/ for diagnosis (the Pi's intermittent labwc
+# clipboard failures are the motivation).
+set -u
+
+REPO="${TABBED_REPO:-$(cd "$(dirname "$0")/.." && pwd)}"
+ITER="${1:-30}"
+shift 2>/dev/null || shift
+
+FAIL_DIR=/tmp/stress-fail
+rm -rf "$FAIL_DIR"
+mkdir -p "$FAIL_DIR"
+
+pass=0
+fail=0
+for i in $(seq 1 "$ITER"); do
+    out=$(cd "$REPO" && python3 -m pytest tests/test_smoke.py -k clipboard -q "$@" 2>&1)
+    rc=$?
+    if [ "$rc" -eq 0 ]; then
+        pass=$((pass + 1))
+        echo "[$(date +%H:%M:%S)] iter $i/$ITER PASS"
+    else
+        fail=$((fail + 1))
+        d="$FAIL_DIR/iter-$i"
+        mkdir -p "$d"
+        printf '%s\n' "$out" >"$d/pytest.txt"
+        cp /tmp/tabtest.log "$d/tabtest.log" 2>/dev/null || true
+        cp /tmp/labwc.log "$d/labwc.log" 2>/dev/null || true
+        echo "[$(date +%H:%M:%S)] iter $i/$ITER FAIL (snapshot in $d)"
+    fi
+done
+
+echo "=== $pass passed, $fail failed (of $ITER) ==="
+[ "$fail" -eq 0 ]