Handle zero-temperature velocity rescaling explicitly

[galjos]

Hard temperature resets, Berendsen coupling, and stochastic velocity rescaling divide by the current temperature. A zero-temperature system can therefore produce non-finite velocities or remain silently stuck below a positive target.

Expected behavior:

• zero-to-zero rescaling keeps finite zero velocities
• positive-target rescaling from zero kinetic temperature fails with a clear velocity-initialization message
• all three rescaling paths behave consistently

[galjos]

Consolidated into #379. The replacement implementation is #385.