feat(builders): add an after-transform observer

[lucamaraschi]

Description

Add an optional onAfterTransform observer to @workflow/builders.

The observer exposes the accepted result of each workflow SWC transform, including:

• Transform mode
• Source and absolute filenames
• Original source
• Generated code
• Per-file workflow manifest

The callback runs only after the transformed manifest has been validated and accepted into the bundle manifest. It is awaited, and throwing from it aborts the build.

The observer cannot replace generated code. A source file may be observed multiple times across transform modes, bundles, and watch rebuilds.

BaseBuilder propagates the observer through step, workflow, and client bundle creation. The API is optional and has no effect on existing builder integrations.

How did you test your changes?

Added focused tests covering:

• The observer payload
• Invocation after manifest acceptance
• Awaiting asynchronous observers
• Aborting the build when an observer throws
• Not observing transforms rejected because of duplicate manifest IDs

Validation performed:

• pnpm --filter @workflow/builders build
• pnpm --filter @workflow/builders typecheck
• pnpm --filter @workflow/builders test
  • 18 test files passed
  • 233 tests passed
• Biome check on all changed files
• git diff --check main...HEAD

PR Checklist - Required to merge

• 📦 pnpm changeset was run to create a changelog for this PR
  • Minor release for the additive @workflow/builders API
• 🔒 DCO sign-off passes
• 📝 Ping @vercel/workflow in a comment once the PR is ready

[changeset-bot]

🦋 Changeset detected

Latest commit: c0005fa

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 16 packages

Name	Type
@workflow/builders	Minor
@workflow/astro	Patch
@workflow/cli	Patch
@workflow/nest	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/nuxt	Patch
@workflow/rollup	Patch
@workflow/sveltekit	Patch
@workflow/vite	Patch
@workflow/vitest	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/core	Patch
@workflow/web-shared	Patch
@workflow/web	Patch

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[vercel]

@lucamaraschi is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[lucamaraschi]

@vercel/workflow ready for review

[VaguelySerious]

@lucamaraschi We require commits to be signed. Could you squash+sign the PR and force-push?

Also, I'm curious what you're using this for! Can you share an example?