Move toju-app into own its folder

toju-app/src/app/shared-kernel/README.md

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+# Shared Kernel
+
+Types and constants that are **intentionally shared** across multiple bounded
+contexts (domains). Changing anything here affects every consumer, so changes
+require coordination.
+
+## Files
+
+| File | Contents |
+|---|---|
+| `user.models.ts` | `User`, `UserStatus`, `UserRole`, `RoomMember` |
+| `room.models.ts` | `Room`, `RoomSettings`, `RoomPermissions`, `Channel`, `ChannelType` |
+| `message.models.ts` | `Message`, `Reaction`, `DELETED_MESSAGE_CONTENT` |
+| `moderation.models.ts` | `BanEntry` |
+| `voice-state.models.ts` | `VoiceState`, `ScreenShareState` |
+| `chat-events.ts` | `ChatEventType`, `ChatEvent`, `ChatInventoryItem` |
+| `media-preferences.ts` | `LatencyProfile`, `ScreenShareQuality`, quality presets |
+| `signaling-contracts.ts` | `SignalingMessage`, `SignalingMessageType` |
+| `attachment-contracts.ts` | `ChatAttachmentAnnouncement`, `ChatAttachmentMeta` |
+
+## When to add here vs. in a domain
+
+Add to shared-kernel when a type is referenced by **two or more domains** or by
+**infrastructure + store**. If a type is only used inside one domain, keep it
+in that domain's `domain/` folder.
+
+## Backward compatibility
+
+`core/models/index.ts` re-exports everything from this folder so existing
+`import { X } from 'core/models'` lines keep working. **New code** should
+import directly from `shared-kernel/` for clarity.