feat(fusion_accounting_assets): inherit account.move.line for asset linkage

- fusion_asset_id Many2one on account.move.line (ondelete='set null':
  invoice line preserved if asset is removed)
- fusion_asset_count compute (smart-button friendly)
- action_open_fusion_asset() returns a window action to jump to the asset
- 3 new tests (66 total)

Made-with: Cursor

fusion_accounting_assets/models/__init__.py

@@ -3,3 +3,4 @@ from . import fusion_asset
 from . import fusion_asset_depreciation_line
 from . import fusion_asset_disposal
 from . import fusion_asset_anomaly
+from . import account_move