Add SOFTSSharp model

[hljubic]

This PR adds SOFTSSharp, an extension of SOFTS that introduces stochastic variable-position encoding inside the STAD aggregation-redistribution component.

SOFTSSharp is intended as an incremental improvement over SOFTS while preserving linear computational complexity.

Reference implementation and ongoing research:
https://github.com/hljubic/SOFTSsharp

Changes:

• Adds the SOFTSSharp model implementation.
• Exposes SOFTSSharp through neuralforecast.models.
• Registers softssharp in the model filename mapping.
• Adds a minimal model test using the existing check_model AirPassengers workflow.

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