[RPG] Does being stunned interrupt concentration

PFSRD > Magic > Duration > Concentration

Concentration

The spell lasts as long as you concentrate on it. Concentrating to maintain a spell is a standard action that does not provoke attacks of opportunity. Anything that could break your concentration when casting a spell can also break your concentration while you're maintaining one, causing the spell to end. See concentration.

You can’t cast a spell while concentrating on another one. Some spells last for a short time after you cease concentrating.

Thus,

• What action am I allowed while I am concentrating? Can I move? Attack? Cast another spell?

It takes your standard action. You can use your other actions as you please, to move or attack (e.g. an attack of opportunity), so long as they don’t require a standard action. The only special exception is spellcasting; you can’t do that even if it’s a swift-action spell.

• Can I decide to stop concentrating at any time?

Somewhat vaguely defined. Certainly, on your turn you can choose not to spend your standard action to concentrate and thus the spell immediately ends. And someone else can forcibly end your concentration outside your turn, by forcing a Concentration check that you fail. Thus, I feel that it would be unreasonable to deny you the ability to end concentration at any time you like.

• Do the effects end immediately on me ending my concentration (either through interruptions or voluntarily)?

Depends on the spell. Precipitate, yes. Other spells have durations like “Concentration + one round,” and those would linger for the indicated amount of time.

Your question is indirectly addressed by these tweets from Jeremy Crawford:

As DM, I'd rule that you can't control a delayed blast fireball while inside an antimagic field. ... You could regain control when neither you nor the bead was in the antimagic field.

Note that he's assumed the Delayed Blast Fireball spell doesn't get suppressed just because the caster is inside an Antimagic Field; however the intention (the rules are silent on this) is that you can't control a spell while you're in the antimagic field or while the spell is suppressed.