docs(fusion_accounting): per-sub-module CLAUDE.md, UPGRADE_NOTES.md, README.md

Task 20 of Phase 0: document the sub-module split.

- fusion_accounting_core: foundation doc covering security groups, shared-field
  schema preservation, and the Enterprise-detection helper.
- fusion_accounting_ai: preserves the original module's AI-specific design
  decisions, Odoo 19 gotchas, deployment commands, controllers, models, theme
  rules, and known issues. Adds a new Data-adapter pattern section documenting
  tri-mode routing (fusion / enterprise / community).
- fusion_accounting_migration: doc for the Enterprise uninstall safety guard
  and the wizard shell that future feature sub-modules will extend.
- fusion_accounting (meta): rewritten CLAUDE.md as a pure overview pointing at
  sub-modules, plus a new README.md covering one-click install/uninstall.

Each sub-module now has CLAUDE.md (Cursor/Claude context), UPGRADE_NOTES.md
(version-by-version deltas / reference sources), and README.md (user-facing
install/usage docs). 11 files total.

Made-with: Cursor

fusion_accounting_core/UPGRADE_NOTES.md

@@ -0,0 +1,28 @@
+# UPGRADE_NOTES — fusion_accounting_core
+
+## V19.0.1.0.0 (initial — Phase 0)
+
+### Reference sources
+- `RePackaged-Odoo/accounting/account_accountant/models/account_move.py` (Enterprise extension fields read for schema match)
+- `RePackaged-Odoo/accounting/account_accountant/models/account_reconcile_model.py` (same)
+
+### Mirror-zone files (none in _core — _core has no Mirror zone)
+
+### Abstract-zone files (all of _core is abstract)
+- `models/account_move.py`
+- `models/account_reconcile_model.py`
+- `models/ir_module_module.py`
+
+### Intentional deltas from Odoo
+- Shared-field declarations have NO compute methods, NO @api decorators beyond
+  basic field types. Enterprise's account_move.py adds compute methods and
+  business logic; we deliberately do not duplicate them. When Enterprise is
+  installed, its compute methods run; when it's not, the fields are simply
+  unused (until a fusion sub-module decides to own that behavior).
+
+### Migrations
+- `migrations/19.0.1.0.0/pre-migration.py` — rehome fusion security xml-ids
+  from module='fusion_accounting' to module='fusion_accounting_core' BEFORE
+  data-load (avoids unique-constraint crash on upgrade from pre-Phase-0)
+- `migrations/19.0.1.0.0/post-migration.py` — idempotent safety-net for the
+  same rehome (zero-op if pre-migration already ran)