fix: ash-compatible arithmetic in _cached_speedtest; clean up speedtest null guards

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package/kit-busrouter/files/usr/lib/busrouter/telemetry-synology.sh

@@ -292,8 +292,8 @@ _collect_wan() {
 			_dl=$(echo "$_st" | awk '{print $1}')
 			_ul=$(echo "$_st" | awk '{print $2}')
 		fi
-		[ -z "$_dl" ] || [ "$_dl" = "0" ] && _dl="0.0"
-		[ -z "$_ul" ] || [ "$_ul" = "0" ] && _ul="0.0"
+		[ -n "$_dl" ] || _dl="0.0"
+		[ -n "$_ul" ] || _ul="0.0"
 	fi
 
 	printf '{"score":%s,"latency_ms":%s,"dl_mbps":%s,"ul_mbps":%s,"jitter_ms":%s,"loss_pct":%s}' \
@@ -416,7 +416,8 @@ _cached_speedtest() {
 	[ -f "$_f" ] || return 1
 	_mtime=$(stat -c %Y "$_f" 2>/dev/null || stat -f %m "$_f" 2>/dev/null)
 	[ -n "$_mtime" ] || return 1
-	_age=$(( $(date +%s) - _mtime ))
+	_st_now=$(date +%s)
+	_age=$(( _st_now - _mtime ))
 	[ "$_age" -lt 2100 ] || return 1
 	cat "$_f"
 }