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_migration/README.md

@@ -0,0 +1,22 @@
+# Fusion Accounting Migration
+
+Transitional helper for moving clients from Odoo Enterprise to Community + Fusion.
+
+## When to use
+
+Install this module ONCE per client during the Enterprise->Fusion switchover.
+After the switchover is complete and the client is comfortable on Community,
+this module can be uninstalled.
+
+## How it works
+
+1. Install fusion_accounting (the meta-module) — pulls in this module
+2. Open Fusion Accounting -> Migrate from Enterprise (top-level menu)
+3. Wizard shows which Enterprise modules are detected and what migrations are available
+4. Run migration; wizard reports counts and warnings
+5. Uninstall Enterprise modules in dep-safe order (the safety guard prevents premature uninstall)
+
+## Override the safety guard
+
+If you need to uninstall an Enterprise module WITHOUT migrating (data will be lost),
+set `fusion_accounting.migration.<module>.completed` to True in System Parameters.