* ci(regression): compute the shard matrix from recorded fixture timings
* ci(regression): refresh shard timings from a green post-PSNR run
* fix(ci): close two silent-skip holes in the shard schedule contract
* ci(regression): schedule the new static-volume-future-set fixture
* test(producer): regenerate static-volume-future-set golden in the pinned container
* fix(producer): accept partial color metadata in plan v2
* test(engine): make partial color probe hermetic
* fix(producer): validate plan v2 sentinels in fallback mode
## What
- publishes Plan v2 content-addressed artifacts directly from the Cloud Run planner's private staging directory to GCS
- commits the Plan v2 manifest only after every referenced artifact is durable
- removes the second local Plan v2 CAS directory and preserves the existing Cloud Workflows wire contract
- exports a reusable GCS publisher adapter
## Why
Cloud Run planner, chunk, and assembler requests are independent containers and do not share a filesystem. The distributed contract must contain only durable GCS locators.
This layer makes the Plan v2 publication path object-store-native. Like the AWS parent PR, it still relies on one planner-local frozen v1 tree inside the producer; eliminating that remaining staging tree requires direct Plan v2 emission in a later layer.
## Design invariants
- local paths never cross a request or worker boundary
- manifest and artifact locators derive from one validated GCS output prefix
- immutable objects use generation-zero conditional creation and exact digest/size verification
- retries may reuse exact objects but never overwrite conflicts
- the manifest is the final publication commit point
- chunk and assembler requests independently download and verify only their target artifacts
## Test plan
- [x] all 95 GCP Cloud Run package tests pass
- [x] package typecheck passes
- [x] package build passes
- [x] changed-file lint, format, fallow, and repository commit gates pass
- [x] end-to-end adapter test covers plan, target-scoped chunk, and assemble through Fake GCS with no shared local directory
Xuanru Li