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/models/fusion_asset_depreciation_line.py

@@ -0,0 +1,19 @@
+"""Per-period depreciation board lines for an asset.
+
+Stub created with Task 8 (so fusion.asset One2many resolves). Fully
+elaborated in Task 9.
+"""
+
+from odoo import fields, models
+
+
+class FusionAssetDepreciationLine(models.Model):
+    _name = "fusion.asset.depreciation.line"
+    _description = "Asset Depreciation Board Line"
+    _order = "asset_id, scheduled_date"
+
+    asset_id = fields.Many2one('fusion.asset', required=True, ondelete='cascade')
+    scheduled_date = fields.Date(required=True)
+    amount = fields.Monetary()
+    currency_id = fields.Many2one(related='asset_id.currency_id', store=True)
+    is_posted = fields.Boolean(default=False)