Prototype Portal has a few bugs

Prototype Portal is broken in the following ways:

- [ ] When you activate the ability, it asks you what you want to pay for x. This doesn't make any sense; x is fixed based on what card(s) are imprinted. There shouldn't be any user choice involved.

- [ ] If you do choose an x, it *adds* that cost to the cost you should have to pay, which is just weird. I.e. if you have a Gilded Lotus imprinted, activating for x=3 charges you 5+3 mana.

- [ ] You can activate the portal without having anything imprinted, which isn't allowed per the gatherer rulings.

- [ ] If you imprint two (or more) artifacts due to an effect like Panharmonicon, activating the portal should require the combined mana costs of the two (or more) artifacts, and make a copy of each of them (https://magicjudge.tumblr.com/post/151433767023/what-happens-when-i-play-prototype-portal-with/embed). Currently, it only charges you for - and creates a copy of - the first artifact, and any further ones get ignored.

Start by locating the Java implementation and tests for Prototype Portal, then reproduce each listed activation case, including multiple imprinted artifacts. Done means activation no longer asks for X, charges the correct combined mana costs, is blocked without an imprint, and creates copies for every imprinted artifact.

Written by the indexing model from the issue text.