Create an RPC extension registry for WebSocket handlers #68

What to build

apps/server/src/ws.ts (1661 lines, already with merge conflicts) inlines every RPC handler in a single switch block. Extract a registry pattern where new features register handlers without modifying ws.ts. Migrate todosLoad / todosMutate as the first consumer to prove the pattern.

Pattern

// Extension modules register themselves:
registerRpcMethod("todos.load", handleTodosLoad);
registerRpcMethod("todos.mutate", handleTodosMutate);

// ws.ts iterates registered handlers instead of a switch block

After migration, ws.ts should have no todo-specific imports or logic.

Acceptance criteria

• RpcExtensionRegistry module exists in apps/server/src/rpc/ with registerRpcMethod(name, handler) API
• ws.ts routes unknown methods to the registry instead of using a monolithic switch
• Todo RPC handlers (todosLoad, todosMutate) migrated out of ws.ts into a standalone module
• vp check passes
• vp test passes
• No behavior change to existing todo panel

Blocked by

• Resolve upstream merge conflicts from t3-code/main fork #67 (Resolve upstream merge conflicts)