feat(fusion_accounting_assets): asset disposal record model

- 4 disposal types: sale, scrap, donation, lost
- mail.thread tracking on type / date / sale amount / partner
- gain_loss_amount computed:
    - sale: sale_amount - book_value_at_disposal
    - scrap / donation / lost: -book_value_at_disposal (full loss)
- ondelete='restrict' on asset_id (cannot delete an asset with disposal)
- move_id placeholder for engine-created journal entry
- 4 new tests (59 total)

Made-with: Cursor

fusion_accounting_assets/models/__init__.py

@@ -1,3 +1,4 @@
 from . import fusion_asset_category
 from . import fusion_asset
 from . import fusion_asset_depreciation_line
+from . import fusion_asset_disposal