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/package.json

@@ -7,7 +7,8 @@
     ".": "./src/index.ts",
     "./payload": "./src/payload/index.ts",
     "./trpc/init": "./src/trpc/init.ts",
-    "./trpc/context": "./src/trpc/context.ts"
+    "./trpc/context": "./src/trpc/context.ts",
+    "./trpc/define-error-middleware": "./src/trpc/define-error-middleware.ts"
   },
   "scripts": {
     "build": "tsc --noEmit",