Add VP freeze/thaw toggle columns to the layer panel when in a paper
space layout. Each viewport gets its own column (VP 1, VP 2, …) showing
a freeze icon per layer. Clicking toggles frozen_layers on the Viewport
entity and syncs back to the panel.
- scene: viewport_list() → [(handle, label, frozen_handles)]
- LayerPanel: VpCol + vp_frozen per layer, sync_with_viewports()
- Message::LayerToggleVpFreeze(layer_idx, vp_col_idx)
- Sync VP columns on layout switch, VPLAYER cmd, undo/redo, layer ops
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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