[State Sync Redesign] Epic 3: UI-controller subscriptions

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Remove the monolithic `metamask` Redux slice via controller-scoped `useSyncExternalStore` subscriptions. Builds on A1's background restructuring (Epics 1 + 2) by adding per-controller UI stores, migrating selectors and components, and removing the slice.

## Key Benefits

- **Scoped concurrent rendering**: Per-controller subscriptions scope React 18 concurrent features (`useDeferredValue`, `useTransition`) to per-controller granularity — deferred or transitioned re-renders evaluate only the affected controller's selectors, not all ~635. (Redux already prevents tearing via react-redux v8, but the monolithic dispatch still evaluates all selectors.) See [#6657](https://github.com/MetaMask/MetaMask-planning/issues/6657).
- **Remove `metamask` Redux slice** and `forceUpdateMetamaskState` (~109 RPC round-trips)
- **8 of 10 circular dependency cycles eliminated**
- **Fine-grained reactivity**: selectors only evaluate when their subscribed controllers change, eliminating fan-out evaluation cost
- **Mechanical migration**: ~670 `useSelector` + ~37 `connect()` via compatibility hook, ~85% automatable
- **Enables `selectors.js` decomposition**: `controllerDeps` annotations provide a natural splitting criterion; slice removal eliminates `selectors.js`'s role as the sole import hub for controller state

## ADR Option

A2 (UI migration)

## Phase

A2 (UI-controller subscriptions) — follows A1

## Dependencies

Epic 1 + Epic 2 (A1 phase must be complete: controller keys preserved in transport, keyed initial state, background cleanup done)

## React 18 sequencing

MVP lands on React 17 (using `use-sync-external-store/shim`). Horizontal expansion benefits from React 18's automatic batching and native `useSyncExternalStore`. No blocking dependency in either direction. On React 18, per-controller subscriptions additionally scope concurrent rendering features (`useDeferredValue`, `useTransition`) to per-controller granularity — deferred or transitioned re-renders evaluate only the affected controller's selectors instead of all ~635. (Tearing is already handled by react-redux v8's internal use of `useSyncExternalStore`; the benefit here is scope reduction.)

## Issues

### MVP (vertical slice)
- [ ] 3.1: Implement `ControllerStateProxy`, `StateSubscriptionService`, and `useControllerState` — #40038
- [ ] 3.2: Create `renderWithControllerState` test utility — #40039
- [ ] 3.3: Migrate one PreferencesController component (MVP validation) — #40040

### Horizontal expansion
- [ ] 3.4: Implement `useControllerSelector` compatibility hook — #40041
- [ ] 3.5: Annotate high-frequency selectors with `controllerDeps` — #40042
- [ ] 3.6: Codemod for `useSelector` → `useControllerSelector` — #40043
- [ ] 3.7: Component migration (per domain, repeating) — #40044

### Slice removal
- [ ] 3.8: Remove `forceUpdateMetamaskState` (~109 call sites) — #40045
- [ ] 3.9: Remove dual-write and `updateMetamaskState` thunk — #40046
- [ ] 3.10: Update Sentry state capture chain — #40047
- [ ] 3.11: Update state logs and debug tools — #40048
- [ ] 3.12: Update Storybook state mocks — #40049
- [ ] 3.13: Remove `metamask` Redux slice — #40050
- [ ] 3.14: Debug-only controller state inspector (optional) — #40051

## Critical path

3.1 → 3.4 → 3.5 → 3.6 → 3.7 → 3.8 → 3.9 → 3.10 → 3.13

## Validation Criteria (Epic complete)

- [ ] `ui/ducks/metamask/metamask.js` deleted
- [ ] `FlattenedBackgroundStateProxy` type removed or repurposed
- [ ] Zero `useSelector` calls reading `state.metamask` remain
- [ ] Zero `forceUpdateMetamaskState` calls remain
- [ ] Zero `connect()` HOCs reading `state.metamask` remain
- [ ] Sentry state capture validated in dev
- [ ] State logs export works
- [ ] 8 of 10 circular dependency cycles eliminated (`yarn circular-deps:check`)
- [ ] `selectors.js` decomposition unblocked: `controllerDeps` annotations provide splitting criterion, `selectors.js` no longer required as sole import hub
- [ ] E2E test suite passes
- [ ] No new circular dependencies introduced

Start with the prerequisite MVP issues, especially #40038 for ControllerStateProxy, StateSubscriptionService, and useControllerState, then follow the critical path through the listed UI migration issues. Use the validation criteria as the definition of done, including removing ui/ducks/metamask/metamask.js, eliminating state.metamask useSelector calls, and passing the circular-dependency and E2E checks.

Written by the indexing model from the issue text.