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
Empty section template plus the doc-update checklist that the
post-Plan-9 follow-up pass will work through (combined with the still-
pending Plan 8 items so docs are written once).
Spec: docs/superpowers/specs/2026-05-06-input-output-unification-design.md §10, R30.
