refactor(producer): add plan v2 artifact publisher

[jrusso1020]

What

Introduces a manifest-last PlanV2ArtifactPublisher seam and routes planV2() through a local publisher. The local implementation hard-links immutable blobs when staging and CAS share a filesystem, with an atomic copy fallback for cross-device or restricted filesystems.

The frozen v1 catalog and manifest construction are now storage-independent, duplicate digests publish once, and the existing synchronous createPlanV2FromV1() compatibility helper remains available.

Why

Plan v2 currently creates a complete v1 staging tree and then copies the retained bytes into a second local CAS tree before cloud upload. This PR is the first direct-emission foundation: it removes the second set of local data blocks on the normal same-filesystem path and establishes the manifest-last publisher contract needed by direct S3/GCS emission.

This does not yet remove the complete v1 staging tree. That remains the next stacked producer change; oversized-plan rollout stays disabled.

How

• separates plan catalog/manifest construction from storage publication
• publishes each unique digest before committing plan.json
• aborts partial publication without exposing a manifest
• hard-links local immutable blobs and atomically renames the completed transport
• preserves manifest bytes, hashes, materialization, and worker execution

Test plan

• Plan v2 focused tests: 11 passed
• Producer typecheck
• Changed-file oxlint and oxfmt
• Fallow, tracked-artifact, and pre-commit gates
• Same-filesystem inode-sharing assertion
• Injected blob failure proves abort occurs and manifest is never committed
• Execution plan updated with the remaining direct-emission stack and rollout gates

[jrusso1020]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• refactor(producer): add plan v2 artifact publisher #2792 👈 (View in Graphite)
• feat(gcp-cloud-run): support plan protocol v2 #2790
• feat(aws-lambda): support plan protocol v2 #2789
• feat(producer): add content-addressed plan v2 #2788
• feat(producer): version distributed plan protocol #2777
• main

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