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/CLAUDE.md

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+# fusion_accounting_core — Cursor / Claude Context
+
+## Purpose
+Foundation for the Fusion Accounting sub-module suite. Owns:
+- Three security groups (User / Manager / Admin) shared across all sub-modules
+- Shared-field-ownership declarations on `account.move` and `account.reconcile.model`
+- Runtime Enterprise-detection helper: `env['ir.module.module']._fusion_is_enterprise_accounting_installed()`
+
+## What lives here
+- `models/account_move.py` — declares Enterprise-extension fields with identical
+  schemas / relation tables. Pure schema-preservation; no business logic.
+- `models/account_reconcile_model.py` — same pattern for `created_automatically`
+- `models/ir_module_module.py` — Enterprise-detection helpers
+- `security/fusion_accounting_security.xml` — privilege + 3 groups + auto-assignment
+
+## Critical rules
+- NEVER add business logic to the shared-field models (account_move.py here).
+  Logic belongs in the feature sub-module that owns it (e.g. fusion_accounting_bank_rec).
+- NEVER rename the relation tables for shared M2Ms. They must match Enterprise verbatim
+  for the dual-ownership pattern to work.
+- Shared fields here have NO defaults beyond what Enterprise sets. The point is preservation.
+
+## Cross-references
+- Parent design: `fusion_accounting/docs/superpowers/specs/2026-04-18-fusion-accounting-enterprise-takeover-roadmap-design.md` (Section 3)
+- Workspace conventions: `/Users/gurpreet/Github/Odoo-Modules/CLAUDE.md`

fusion_accounting_core/README.md

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+# Fusion Accounting Core
+
+Foundation module for the Fusion Accounting suite.
+
+## What it does
+
+- Defines three security groups: Fusion Accounting User / Manager / Administrator
+- Auto-promotes Odoo `account.group_account_user` -> Fusion User and
+  `account.group_account_manager` -> Fusion Admin
+- Declares schema-preservation fields on `account.move` and `account.reconcile.model`
+  so that Enterprise extension fields (deferred revenue links, signing user, etc.)
+  survive an Enterprise uninstall
+- Exposes the helper `env['ir.module.module']._fusion_is_enterprise_accounting_installed()`
+
+## Install
+
+This module never installs alone. Install `fusion_accounting` (the meta-module)
+or any of the feature sub-modules — they all depend on `fusion_accounting_core`.
+
+## Uninstall
+
+Uninstalling `fusion_accounting_core` will remove the security groups and the
+schema-preservation fields. If Enterprise is also installed, uninstalling
+`fusion_accounting_core` will cause Odoo to consider the deferred / signing
+fields owned only by Enterprise — which is the original Enterprise-only state
+(no data loss, just back to Enterprise-controlled schema).
+
+## Troubleshooting
+
+If users are missing the "Fusion Accounting" privilege section in user settings
+after install, the `implied_ids` mechanism only fires for newly-added users.
+Backfill existing users via SQL:
+
+    INSERT INTO res_groups_users_rel (gid, uid)
+    SELECT g.res_id, gu.uid
+    FROM res_groups_users_rel gu
+    JOIN ir_model_data g ON g.module = 'fusion_accounting_core' AND g.name = 'group_fusion_accounting_user'
+    JOIN ir_model_data ag ON ag.module = 'account' AND ag.name = 'group_account_user' AND gu.gid = ag.res_id
+    ON CONFLICT DO NOTHING;

fusion_accounting_core/UPGRADE_NOTES.md

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+# 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)