Add piecewise diffusion sampler for downscaling

[VeraChristina]

Is your feature request related to a problem? Please describe.

For diffusion downscaling we need a sampling strategy, where sampling should:

• traverse the high-noise regime quickly;
• spend most computation in the low-noise regime where fine-scale fidelity is determined.

Describe the solution you'd like

Introduce a piecewise sigma schedule:

• rapid collapse through the high-noise regime;
• Karras-style refinement in the low-noise regime.

This is intended as a purely additive change consisting of one new sampling schedule in models/src/anemoi/models/transport/schedules.py

Describe alternatives you've considered

No response

Additional context

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Organisation

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