xP: fix command sequence number generation

xP/public/xP.js

@@ -110,8 +110,9 @@ class RelayRpc extends EventTarget {
 		if (typeof params !== 'object')
 			throw "Method parameters must be an object"
 
+		// Left shifts in Javascript convert to a 32-bit signed number.
 		let seq = ++this.commandSeq
-		if (seq >= 1 << 32)
+		if ((seq << 0) != seq)
 			seq = this.commandSeq = 0
 
 		this.ws.send(JSON.stringify({commandSeq: seq, data: params}))