User code such as `new Worker(new URL('./w.ts', import.meta.url))` reads
`process.env.NEXT_DEPLOYMENT_ID` from inside the worker scope, but vinext
only defined the internal `__VINEXT_DEPLOYMENT_ID` mirror — leaving the
public Next.js identifier as `undefined` in every browser-side bundle.
Mirror Next.js' webpack DefinePlugin behavior (see
packages/next/src/build/define-env.ts) and statically inline the value
into the global Vite `define`. The same map is applied to client, SSR,
RSC and Vite worker bundles, so workers transparently see the deployment
id without needing a globalThis bridge (which can't be shared across
the main-thread / worker boundary anyway).
When no deploymentId is configured, inline `false` — the same fallback
Next.js uses, so feature checks like `if (process.env.NEXT_DEPLOYMENT_ID)`
remain dead-code eliminated.
James Anderson