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I am not a physicist, but I thought that it is generally accepted that decoherence does not solve the measurement problem. It explains why quantum effects are not observable in a noisy environment but still does not explain the non-unitarity of measurements.

It does not solve the measurement problem, but it does explain why you don't see quantum phenomena such as superposition or tunneling at the classical limit.

Even lasers deal with weird temperatures: they're negative.

This doesn't answer the question. We know how classical physics emerges from a large number of quantum events through statistics. That was not the point of Schrödinger's cat though experiment! The idea is to link a macroscopic event to a random subatomic event that is in a superposition. When or how does the wave function collapse, does it even collapse?

How is thermal noise different from heat?