refactor!: strip Clustering to minimal wrapper

[FBumann]

Summary

Builds on #654. Further strips the Clustering class to a minimal wrapper around tsam_xarray.

• Remove ClusteringPlotAccessor entirely (~370 lines) — users can plot with tsam_xarray's DataArrays directly
• Remove original_data, aggregated_data, _metrics from Clustering — no longer stored or serialized
• Remove include_original_data parameter from to_dataset()/to_netcdf()
• Remove build_metrics() from _ReducedFlowSystemBuilder
• Simplify _create_reference_structure() to just serialize ClusteringResult.to_dict() + original_timesteps
• Move xr and np imports to TYPE_CHECKING (no longer needed at runtime in base.py)

base.py: 1633 → 393 lines (-76%)

Test plan

• All clustering tests pass (213)
• CI

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