Shared core

One source of truth for Express/NestJS/Next.js

src/shared/ is the single source of truth:

Module	Role
`errors`	`TranslationError`
`headers`	`redactHeaders`
`multipart-node`	`collectMulterFiles`, `parseMultipart` (multer-aware + native `Request.formData`)
`write-node` / `write-web`	`writeRuntimeResponse` with drain/backpressure + `iterator.return()` on disconnect
`translate-node` / `handler-node`	`toRuntimeRequest` / handler bridge

Fix once, ship everywhere. src/shared/ is not a published package — not exported to consumers.