Well, lots of points now as the thread drifts a little. My fault for most of it and I apologize. Moderators, if you want to cancel this post or move it to a new thread, I will understand and not question it.
Jim and RM9792,
I understand your decision not to stop for anyone when the alarm sounds. That is your right and, as I pointed out, it is not illegal to leave from a citizen attempting arrest. I posted the warning just to ensure that anyone considering it made an informed decision which it looks like you both did.
As for the law authorizing the detention and force, we seem to be looking at the Penal Code quite a bit, especially chapter 9. This is good for understanding part of the problem, but it is not the complete question. There are other laws we need to look at, and specifically in this case one is the Code of Criminal Procedure. The CCP has the authorities for arrest in it. Most of them are under Chapter 14, but the one I referred to for this is in chapter 18.
Chapter 14 gives all citizens the authority to arrest for a breach of the peace OR a felony that occurs in their presence or view. Note that this is not all misdemeanors, but many of them can be classed as a breach of the peace. The term is not legally defined anywhere I know of and most commonly is used for things like assaults or fights. There is also a group that believes almost anything in Chapter 42 or 43 is a breach of the peace, based on the title (Title IX) being called offenses against the public order. An argument can be made for most of Chapter 42, with things like disorderly conduct or riot, but I have trouble with claiming silent calls to 911 is a breach of the peace. I would go with anything other than a felony had better be violent or disturbing a group of people before I would recommend a citizen's arrest.
But, we also have Article 18.16 of the CCP. This article is about preventing the consequences of a theft. It gives ANY citizen the authority to recover any stolen property and requires that if they do, the property and the suspect be taken to a magistrate if the suspect can be caught. It does allow you to turn the suspect and property over to a peace officer for this purpose. Note that this is any theft, including misdemeanors. Also note, the specific wording is to say reasonable grounds for the belief that the property is stolen. It does not even have to rise to the level or probable cause, and the Constitution would only apply to the police later in the process.
So, IMHO, what this law does is allow for a store employee to detain you for further investigation if they reasonably suspect you are stealing. I still think most citizens in this state, in other words the jury, would find the suspicion reasonable if you are leaving the store and the alarm goes off. Thus, I feel the law would allow for the citizen to stop you and detain you, including the use of force necessary for an arrest under Chapter 9 of the PC, if the alarm goes off. I think most juries would see it as reasonable. I also think your refusing to stop for any questioning would be seen as unreasonable by the jury.
Obviously, I could be wrong on this interpretation of the law or what the jury would see as reasonable. I post this just so you will know to look at all of the laws concerning this and then make an informed decision on how you want to act. As I said before, I think most stores would have policies against the employee using force to stop you, but there have been cases where store employees acted outside of their company policy too.
One final point, which is really irrelevant to this but is a technical correction. Theft under 1500 is not always a misdemeanor. There are certain items which are always a felony (such as parts of livestock - so don't shoplift a steak

) or a firearm (see, I can keep it somewhat related). One of the interesting things is that a 25 cent peice of gum can be a felony, if the person has been twice convicted of theft of any value before. Three sticks of gum on three occasions gets a shoplifter into felony range.