Has the Westminster Parliament agreed to dissolve the UK on the basis of a 50.1% "yes" vote?
It would not mean the 'dissolution of the UK', since without Scotland there would still be a United Kingdom of England, Wales and Northern Ireland. That aside, the answer to your question is essentially 'yes': a simple majority would trigger the independence process. However, everything about that process, including the timetable, would be up for negotiation between the Scottish and UK governments, and the necessary legislation could not be put before the UK Parliament until those negotiations reach agreement. The 'Yes' campaign insists the negotiation would be short and straightforward. That seems optimistic, to put it mildly, given the complexity of the issues.