refactor(core-trpc): make the tRPC context router-agnostic

Ports the type-safety/ergonomics half of the upstream cycle-break: core-trpc
no longer imports AppRouter from @repo/core-api (an illegal
core -> core-composition boundary edge). The runtime context and both
providers are now AnyTRPCRouter-generic; consumers inject their router type
via useTRPC<TRouter>().

The clean-slate tree is already cycle-free (no kept feature consumes
core-trpc), so this is purely the architecture/boundary improvement. The
per-feature app-slice types the upstream commit added for the demo features
are intentionally skipped: no kept feature consumes useTRPC yet.

Also covers the router-agnostic useTRPC delegation: the refactor turned
useTRPC into a wrapper function whose body was unexercised, dropping
client.ts below the coverage floor — client.test.ts stubs the context
factory and asserts the delegation, restoring client.ts to 100%.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016j8z4VHjedXDTjEDNg7qHK

packages/core-trpc/src/providers/tanstack-provider.tsx

@@ -4,7 +4,7 @@ import { useState } from "react";
 import { QueryClientProvider } from "@tanstack/react-query";
 import { createTRPCClient, httpBatchLink } from "@trpc/client";
 import superjson from "superjson";
-import type { AppRouter } from "@repo/core-api";
+import type { AnyTRPCRouter } from "@trpc/server";
 import { TRPCProvider } from "../client";
 import { getQueryClient } from "../query-client";
 
@@ -17,7 +17,9 @@ export function TanstackTrpcProvider({
 }) {
   const [queryClient] = useState(() => getQueryClient());
   const [trpcClient] = useState(() =>
-    createTRPCClient<AppRouter>({
+    // Router-agnostic on purpose: procedure typing comes from useTRPC<TRouter>
+    // at the call sites, not from the wire-level client.
+    createTRPCClient<AnyTRPCRouter>({
       links: [httpBatchLink({ url: trpcUrl, transformer: superjson })],
     }),
   );