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/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Assets',
-    'version': '19.0.1.0.10',
+    'version': '19.0.1.0.11',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented asset management with depreciation schedules.',
     'description': """