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/define-error-middleware.ts

@@ -0,0 +1,36 @@
+import { TRPCError } from "@trpc/server";
+import type { TRPC_ERROR_CODE_KEY } from "@trpc/server/rpc";
+import { t } from "./init";
+
+type ErrorCtor = new (...args: never[]) => Error;
+
+/**
+ * Build a tRPC middleware that translates domain errors to TRPCError.
+ *
+ * Each tuple pairs a constructor with a TRPC error code. The middleware
+ * runs the procedure body inside a try/catch; on `instanceof Ctor`,
+ * it throws a TRPCError with the configured code and the original error
+ * preserved as `.cause`. Unmapped errors propagate untouched (tRPC's
+ * default INTERNAL_SERVER_ERROR handling applies).
+ *
+ * Owned by features: each feature passes its own constructors in.
+ * core-shared never enumerates feature-specific error classes.
+ */
+export function defineErrorMiddleware(
+  map: ReadonlyArray<readonly [ErrorCtor, TRPC_ERROR_CODE_KEY]>,
+) {
+  return t.middleware(async ({ next }) => {
+    const result = await next();
+    if (!result.ok) {
+      const cause = result.error.cause;
+      if (cause instanceof Error) {
+        for (const [Ctor, code] of map) {
+          if (cause instanceof Ctor) {
+            throw new TRPCError({ code, message: cause.message, cause });
+          }
+        }
+      }
+    }
+    return result;
+  });
+}