Fix the release guard in a particle pool

Your particle system reuses particles from a pool. A collision handler released the same particle twice, so the pool held it twice and later handed one particle to two effects at once. The fix makes a release count only for a particle that is currently checked out.

An assistant wrote run_pool_ops(ops). It is in the editor. It has a bug. Find it and fix it.

Input and output:

• ops: the pool operations in order, each "get" or "release:<id>".
• Return the id each "get" hands out, in order.

Rules the tests check:

• A get takes the most recently released particle (last in, first out). If none is free, it creates a new particle. New ids are 1, 2, 3 and so on, in creation order.
• A particle is checked out from the get that hands it out until a release of its id.
• "release:<id>" needs one or more ASCII digits after the colon, read as a number. A release puts the particle back in the pool only if it is checked out right now. Any other release does nothing, including a second release and a release of an id the pool never handed out.
• Any other op is ignored.