refactor!: reapply clustering dim-rename layer drop via tsam-xarray DimNames (#739)

[FBumann]

What

Revert of the revert: reapplies #739 (dropped clustering dim-rename layer, DimNames(period='original_cluster'), tsam_xarray >= 0.6.4) on top of main after #746 removed it.

⚠️ DO NOT MERGE until v8.0.0

#739 carries a BREAKING CHANGE for persisted clustering artifacts — JSON/netCDF saved by flixopt ≤ 7.2.x no longer load. Per the policy in #746/#747, main stays fully v7-compatible; this change ships with v8.0.0.

This PR exists as the tracked home for the reapplication on main. It becomes redundant if the v8 integration branch (#747, which already contains #739) lands first — in that case, close it.

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