feat(fusion_accounting_assets): main fusion.asset model with state machine

- fusion.asset: lifecycle (draft -> running -> paused -> disposed)
- mail.thread + mail.activity.mixin tracking
- 3 depreciation methods + 3 prorate conventions selections
- monetary cost / salvage with check constraints (models.Constraint)
- computed book_value, total_depreciated, last_posted_date
- action_set_running / pause / resume / set_draft transitions
- minimal stubs for fusion.asset.category and
  fusion.asset.depreciation.line so the One2many / Many2one comodels
  resolve at registry build time; expanded in Tasks 9 + 10
- 7 new tests (47 total)

Made-with: Cursor

fusion_accounting_assets/__manifest__.py

@@ -1,6 +1,6 @@
 {
     'name': 'Fusion Accounting Assets',
-    'version': '19.0.1.0.5',
+    'version': '19.0.1.0.6',
     'category': 'Accounting/Accounting',
     'summary': 'AI-augmented asset management with depreciation schedules.',
     'description': """
@@ -29,6 +29,7 @@ menu hides; the engine + AI tools remain available for the chat.
         'fusion_accounting_core',
         'fusion_accounting_ai',
         'account',
+        'mail',
     ],
     'data': [
         'security/ir.model.access.csv',