feat: Add logic to copy prognostics from forecaster to downscaler at overlapping timesteps

[MicheleCattaneo]

Adds an optional opt-in copy_prognostic_from_forecaster flag for temporal downscalers.
When enabled on a downscaler run that has a forecaster parent, overwrite the prognostic variables at the overlap steps in the downscaler's GRIB with the forecaster's values, so forecaster-alone and forecaster+downscaler metrics agree at multiples of the forecaster stride. Diagnostics (typically tp) are left untouched. It's going to be off by default, maintaining the current behavior.

Everything is defined from the downscaler's anemoi.json:

• Overlap detection: config.training.explicit_times ({input:[0,6], target:[1..6]}): overlap_offsets = set(input) ∩ set(target) (={6}), window stride = max(input) (=6); global overlap steps = multiples of the stride within the run's steps. If the intersection is empty → log "no overlap" and no-op.
• Detect prognostic list: data_indices.data.data.output.prognostic. output.diagnostic (such as ['tp']) is never copied.

TODO:

• Config flag + validation
• GRIB patching script (earthkit-data, patching both output streams, using model's metadata)
• Snakemake wiring
• End-to-end verification
• Tests (maybe?)

[MicheleCattaneo]

As discussed with @hlasco, we will try to move the logic in an anemoi plugin. This PR will then mostly reduce to a config change and an added dependency on the plugin for the inference environment.
Please ignore the proposed changes for now!

[MicheleCattaneo]

MeteoSwiss/anemoi-plugins-meteoswiss#33

Introduces the plugin