[State Sync Redesign 3.14] Debug-only controller state inspector (optional)

Nobody has claimed this yet.

Redux DevTools has been non-functional in the extension for some time. This is an opportunity to build something better.

**Option A:** Development-only subscriber that writes all controller state changes to a Redux store for Redux DevTools.
**Option B:** Custom `ControllerStateManager` inspector (log diffs, subscribe to specific controllers, query snapshots from console).

## Steps

1. In development builds, expose `window.__controllerStateManager__` for console access
2. Add `getSnapshot(controllerName)`, `subscribe(controllerName, cb)` to the global
3. Optionally connect to Redux DevTools via a debug Redux store
4. Gate behind `process.env.METAMASK_DEBUG`

## Files

- `ui/store/debug-store.ts`

## Dependencies

Issue 3.1 (can proceed in parallel with migration, but not on critical path)

## Context

Part of [State Sync Redesign — Epic 3: UI-controller subscriptions]. Optional/deferred.

Start with ui/store/debug-store.ts and review the state-sync context around Issue 3.1 before choosing between the Redux DevTools subscriber and the custom ControllerStateManager inspector. Done should include development-only console access to the named snapshot and subscription operations, gated by METAMASK_DEBUG; Redux DevTools integration is explicitly optional.

Written by the indexing model from the issue text.