feat(core-shared): add defineErrorMiddleware factory + export t

Factory takes [[ErrorCtor, TRPC_CODE], ...] tuples and returns a tRPC
middleware that translates matching domain errors to TRPCError. Discrim-
inates by instanceof; preserves original error as cause; unmapped
errors propagate (tRPC then wraps them as INTERNAL_SERVER_ERROR with
the original error as .cause — middleware does not interfere).

core-shared never enumerates feature errors — each feature passes its
own constructors in via integrations/api/procedures.ts (Tasks 3-7).

Also exports the `t` instance from trpc/init.ts so feature procedure
files can do t.procedure.use(...).

Also fixes tsconfig.json: rootDir set to "." and @/* path alias added
so test files using @/ resolve correctly under tsc --noEmit.

Refactor log: §1, §2, §4
Spec: R13–R17

packages/core-shared/src/trpc/init.ts

@@ -1,7 +1,7 @@
 import { initTRPC } from "@trpc/server";
 import superjson from "superjson";
 
-const t = initTRPC.create({
+export const t = initTRPC.create({
   transformer: superjson,
 });