docs(design): add feature slice design (#356)

internal/design/feature-slice-design.md

@@ -0,0 +1,171 @@
+---
+status: draft
+date: 2025-01-29
+---
+
+# Feature Slice Design
+
+## Problem
+
+Primitives don't know which preset the user chose:
+
+```tsx
+// Inside @videojs/react - shipped to users
+export function PlayButton() {
+  const player = usePlayer(); // What type is this?
+  // User will use custom feature configuration
+  // We don't know if `playbackFeature` is included
+}
+```
+
+They need:
+
+1. Loosely typed access to the player
+2. A way to check if a feature exists
+3. Type narrowing when the feature is present
+
+## Solution
+
+A `FeatureSlice` provides typed, scoped access to a feature's state and requests. Two API layers:
+
+1. **Primitive** — generic, explicit store/feature
+2. **Player-specific** — typed by registry, uses context
+
+## Design
+
+### Feature Definition
+
+Features have a typed `name` (literal type preserved):
+
+```ts
+const playbackFeature = createFeature<HTMLMediaElement>()({
+  name: 'playback',  // Type: 'playback' (literal)
+  initialState: { paused: true },
+  request: { play, pause },
+});
+```
+
+### Store API
+
+```ts
+// By reference — typed from feature
+store.getFeature(playbackFeature);
+
+// By name — typed from store's features array
+store.getFeature('playback');
+```
+
+Returns `FeatureSlice | undefined`. Flat access to state and requests:
+
+```ts
+const playback = store.getFeature(playbackFeature);
+
+if (!playback) return null;
+
+playback.paused;          // State (flat)
+playback.play();          // Request (flat)
+playback.subscribe(cb);   // Scoped subscription
+```
+
+### Primitive Layer (`@videojs/store`)
+
+Generic APIs that require explicit store/feature:
+
+```ts
+// React — explicit store + feature
+const playback = useFeature(store, playbackFeature);
+
+// Lit — explicit store + feature
+#playback = new FeatureController(host, store, playbackFeature);
+```
+
+### Player Layer (`@videojs/core/dom`, `@videojs/react`, `@videojs/html`)
+
+Typed by `PlayerFeatureRegistry`, uses context:
+
+```ts
+// Registry (manually maintained)
+interface PlayerFeatureRegistry {
+  playback: typeof playbackFeature;
+  volume: typeof volumeFeature;
+  time: typeof timeFeature;
+}
+
+// Utility
+getPlayerFeature(store, 'playback');  // Typed
+
+// React — uses player context
+usePlayerFeature('playback');  // Typed, store from context
+
+// Lit — uses player context
+new PlayerFeatureController(host, 'playback');  // Typed
+```
+
+### Usage Examples
+
+**React primitive (custom feature):**
+
+```tsx
+function CustomControl() {
+  const store = useStore();
+  const custom = useFeature(store, myCustomFeature);
+  // ...
+}
+```
+
+**React player-specific:**
+
+```tsx
+function PlayButton() {
+  const playback = usePlayerFeature('playback');
+  
+  if (!playback) return null;
+
+  // ...
+}
+```
+
+**Lit player-specific:**
+
+```ts
+class PlayButton extends ReactiveElement {
+  #core = new PlayButtonCore();
+  #playback = new PlayerFeatureController(this, 'playback');
+  
+  protected override update(changed: PropertyValues) {
+    super.update(changed);
+    applyElementProps(this, this.#core.getProps(this.#playback.value));
+  }
+}
+```
+
+## FeatureSlice
+
+A thin lens over the store — flat access, scoped subscription:
+
+- State keys exposed directly (not `.state.paused`)
+- Request keys exposed directly (not `.request.play()`)
+- `subscribe()` for scoped updates (reserved name)
+
+## Rationale
+
+1. **Type safety** — Types flow from feature reference or registry
+2. **No collisions** — Features keyed by symbol (unique)
+3. **Layered API** — Primitive layer is generic, player layer is typed and convenient
+4. **Familiar pattern** — Similar to Jotai atoms, Vue InjectionKey
+
+## Alternatives Considered
+
+See [`rfc/player-api/primitives.md`](../../rfc/player-api/primitives.md) for the proxy-level API.
+
+## Files
+
+| Package | File | Change |
+|---------|------|--------|
+| `store` | `src/core/feature-slice.ts` | New — `FeatureSlice` class |
+| `store` | `src/core/store.ts` | Add `getFeature()` |
+| `store` | `src/react/use-feature.ts` | New — `useFeature` hook |
+| `store` | `src/lit/feature-controller.ts` | New — `FeatureController` |
+| `core/dom` | `src/store/registry.ts` | New — `PlayerFeatureRegistry` |
+| `react` | `src/use-player-feature.ts` | New — `usePlayerFeature` hook |
+| `html` | `src/player-feature-controller.ts` | New — `PlayerFeatureController` |