Fix copper layer stackup ordering

[PoeAlex]

Motivation

• The previous layer detection built copper_ids from enabled layers and then sorted them, which could place B.Cu not at the last index and cause incorrect top/bottom mapping and physics at outer faces.
• Need to preserve the actual stackup order provided by modern KiCad APIs so top/bottom indices are correct for plotting and cooling.

Description

• Prefer stackup-derived sequence by calling GetLayerStackup/GetStackup and iterating GetCopperLayers/GetLayerSequence/GetLayers when available to preserve real stack order.
• Add an add_unique helper to de-duplicate layer IDs while preserving discovered order and ensure layers are enabled and pcbnew.IsCopperLayer before inclusion.
• Provide fallbacks to GetCopperLayerCount and constants like pcbnew.F_Cu, pcbnew.In1_Cu, pcbnew.B_Cu, and finally a legacy numeric scan over layer IDs for older KiCad versions.
• Rebuild layer_names from the final ordered copper_ids so plotting and top/bottom references are correct.

Testing

• No automated tests were executed because the plugin environment / KiCad API is not available in CI for this change.

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Codex Task