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/CLAUDE.md

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+# fusion_accounting_migration — Cursor / Claude Context
+
+## Purpose
+Transitional sub-module that helps clients move from Odoo Enterprise accounting
+to Odoo Community + Fusion Accounting without losing data.
+
+## What it does
+- Safety guard: blocks uninstall of Enterprise accounting modules until the
+  migration wizard has run (per-module flag in ir.config_parameter)
+- Migration wizard: shell that other fusion sub-modules extend with per-feature
+  migration logic (Phase 0 ships only the shell)
+
+## Critical
+- The safety guard overrides `button_immediate_uninstall` AND `module_uninstall`
+  on `ir.module.module`. Both paths must be guarded — UI uninstall, CLI uninstall,
+  and API uninstall all go through one or the other.
+- Each fusion feature sub-module that replaces an Enterprise feature MUST extend
+  the migration wizard's `action_run_migration` to add its own migration step
+  AND set the corresponding `fusion_accounting.migration.<module>.completed`
+  flag to True after running.

fusion_accounting_migration/README.md

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+# 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.

fusion_accounting_migration/UPGRADE_NOTES.md

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+# UPGRADE_NOTES — fusion_accounting_migration
+
+## V19.0.1.0.0 (initial — Phase 0)
+
+Skeleton: safety guard + wizard shell. No per-feature migration logic registered yet.
+
+Added by future phases:
+- Phase 1: bank-rec migration (verifies account.partial.reconcile rows are intact; sets `account_accountant.completed` flag)
+- Phase 5: account_followup migration
+- Phase 6: account_asset, account_budget migration