fix(worker): apply build.target to worker bundle

[jaehafe]

Description

Worker bundles silently ignored build.target because the worker pipeline did not thread target into the rolldown() call. The main bundle in build.ts mirrors options.target into transform.target, but plugins/worker.ts skipped this step entirely.

As a result, ES2021+ syntax (logical assignment ??=, &&=, ||=, etc.) leaked into worker output even when the user explicitly set build.target: 'es2020'. On platforms like iOS WebView or older Android, this surfaces as a silent SyntaxError at runtime.

Fix

Mirror the existing transform.target threading from build.ts (lines 638–646) inside bundleWorkerEntry. The worker bundle now respects the resolved build.target for the worker environment, with rollupOptions.transform still able to override per the user's intent.

const workerBuildTarget = workerEnvironment.config.build.target
const bundle = await rolldown({
  // ...
  transform: {
    target: workerBuildTarget === false ? undefined : workerBuildTarget,
    ...rollupOptions.transform,
  },
  // ...
})

Reproduction

Minimal config:

// vite.config.ts
export default defineConfig({
  build: { target: 'es2020' },
  worker: { format: 'es' },
})

Worker source containing any ES2021+ syntax (e.g. let a; a ??= 1):

• Before: ??= is preserved in the worker output despite build.target: 'es2020'.
• After: Output is correctly downleveled to ES2020.

Verified end-to-end against a real-world Vite 8 + Capacitor (iOS) project where the worker chunk previously contained 5× ??=, 7× &&=, 27× ||=, all of which are gone after the fix (worker chunk hash also changes, confirming bytewise different output).

Alternatives considered

• Fixing the workerResolved merge order in config.ts so worker plugin config() returns can override oxc/esbuild for workers. This is a related but separate issue (worker plugins still cannot override target on a per-worker basis). The minimal fix here addresses the more fundamental bug — build.target was completely silenced for workers regardless of any config layer.
• Calling convertEsbuildConfigToOxcConfig for workerConfig.esbuild. This would not have helped, because the worker bundler never read from oxc/esbuild config in the first place — the target option was simply not passed to rolldown.

Tests

Existing worker playground tests (playground/worker/__tests__/*) all continue to pass locally. I did not add a dedicated regression test in this PR — happy to add one in playground/worker/__tests__/ (e.g. a target subdirectory verifying that a worker built with build.target: 'es2020' does not contain ??= in output) if reviewers prefer. Pointing me at a preferred location/style would be helpful.

Reviewer attention

• The transform.target precedence: worker rollupOptions.transform (if set) still wins, matching build.ts. Want to confirm this matches the intended override semantics.
• Whether build.ts's define (e.g. process.env.NODE_ENV handling) should also be threaded for workers. I kept this PR minimal to the target bug; happy to extend if appropriate.

[sapphi-red]

Thanks!

• Whether build.ts's define (e.g. process.env.NODE_ENV handling) should also be threaded for workers. I kept this PR minimal to the target bug; happy to extend if appropriate.

I'd appreciate if you can also send a PR for this one 🙏