Enforce single-instance in socktop-gestures instead of documenting it

The documented restart was `pkill -x lisgd && socktop-gestures &`. pkill exits
non-zero when it matches nothing, so `&&` short-circuits and the daemon
silently does not start when none was running -- you swipe, get nothing, and
reasonably blame the config change you just made. socktop-gestures exec'd lisgd
with no guard, so nothing downstream caught it.

Rather than only fixing the one-liner, move the invariant into the tool: it now
exits non-zero if a daemon is already running, and --replace stops the old one
first (waiting for it to actually exit). This is the failure the README itself
flags as gotcha 3, so a false success there is expensive.

Verified against a live daemon: refusal, --help, bad-option exit codes, and a
clean pid handover under --replace with the carousel still working.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

socktop-gestures

@@ -25,7 +25,48 @@ SWIPE_CMD=${SWIPE_CMD:-$(dirname "$0")/socktop-swipe}
 [ -x /usr/local/bin/socktop-swipe ] && SWIPE_CMD=/usr/local/bin/socktop-swipe
 
 verbose=
-[ "${1:-}" = "-v" ] && verbose=-v
+replace=no
+for a in "$@"; do
+	case "$a" in
+	-v) verbose=-v ;;
+	--replace) replace=yes ;;
+	-h | --help)
+		echo "usage: socktop-gestures [-v] [--replace]"
+		echo "  -v         log each detected gesture"
+		echo "  --replace  stop an already-running daemon first"
+		exit 0
+		;;
+	*)
+		echo "socktop-gestures: unknown option '$a'" >&2
+		exit 2
+		;;
+	esac
+done
+
+# Single-instance guard. lisgd does not grab the input device exclusively, so a
+# second copy makes every swipe fire twice -- the carousel appears to skip panes.
+# Enforced here rather than left to the caller: the obvious restart one-liner
+# `pkill -x lisgd && socktop-gestures` silently starts NOTHING when no daemon was
+# running, because pkill exits non-zero when it matches nothing.
+running=$(pgrep -x -u "$(id -u)" lisgd 2>/dev/null || true)
+if [ -n "$running" ]; then
+	if [ "$replace" = yes ]; then
+		pkill -x -u "$(id -u)" lisgd || true
+		n=0
+		while pgrep -x -u "$(id -u)" lisgd >/dev/null 2>&1 && [ "$n" -lt 30 ]; do
+			n=$((n + 1))
+			sleep 0.1
+		done
+		if pgrep -x -u "$(id -u)" lisgd >/dev/null 2>&1; then
+			echo "socktop-gestures: existing lisgd would not exit" >&2
+			exit 1
+		fi
+	else
+		echo "socktop-gestures: lisgd is already running (pid: $(echo "$running" | tr '\n' ' '))" >&2
+		echo "  Two instances make every swipe fire twice. Use --replace to restart it." >&2
+		exit 1
+	fi
+fi
 
 if [ ! -e "$TOUCH_DEV" ]; then
 	echo "socktop-gestures: $TOUCH_DEV not present" >&2