feat(px): add StormCastEurope prognostic model

[gertln]

Description

Adds StormCastEurope, an autoregressive prognostic downscaling model for the European domain. Given the current state and the target-time ERA5 fields, it predicts the next hourly state, p(REA_{t+1} | REA_t, ERA5_{t+1}) and rolls out autoregressively. It is available at two resolutions: rea6 (COSMO-REA6, roughly 6 km) and rea2 (COSMO-REA2, roughly 2.2 km). It shares its variable channels with the CorrDiff-COSMO Europe downscaler, so the two can be used on the same fields, CorrDiff for single-shot downscaling and StormCastEurope for hourly temporal consitent rollouts.

ERA5 conditioning is supplied through any DataSource or ForecastSource; the wrapper regrids and normalizes ERA5 onto the COSMO grid and concatenates it with the previous state as the diffusion condition. Physical constraints are optional and applied in physical space each step (per-variable bounds together with a shortwave solar gate), and passing constraints=None runs the model unconstrained. Extended static invariants (elevation, land fraction, and latitude/longitude encodings) are loaded from the package.
Hub-height wind is an optional, output-only derivation that is emitted but never fed back into the rollout. Sub-domain inference is available through set_domain(...), which crops to a latitude/longitude bounding box with patch alignment and a NATTEN minimum-domain check.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.
• The CHANGELOG.md is up to date with these changes.
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Dependencies

Adds the stormcast-europe extra (earth2studio[cosmo] + nvidia-physicsnemo>=2.2.0a0 + scipy>=1.15.2), added to all. Like cosmo/stormcast-conus, PhysicsNeMo is git-pinned via [tool.uv.sources] until the natten2d_rope (RoPE) operator ships on PyPI.

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