feat: add product capture compute step

[intel352]

Summary

• add step.compute_product_capture for typed workflow-compute product-capture workloads
• flatten accepted proof result_preview fields into step output for ProductSnapshot consumers
• update manifest/docs/spec and bump workflow-compute to the merged result-preview support

Verification

• GOWORK=off go test ./...
• GOWORK=off wfctl validate workflow.yaml

Depends on workflow-compute PR #112, merged at dc0b7c8.

[Copilot]

Pull request overview

Adds a first-class compute step for workflow-compute’s typed product-capture workload, updates the plugin manifest/docs accordingly, and exposes proof result_preview fields to downstream consumers.

Changes:

• Introduces step.compute_product_capture to submit a typed product-capture URL workload and wait for an accepted proof.
• Extends proof output handling to include result_preview and (additionally) flatten preview fields into the step output.
• Updates plugin manifest/docs/tests and bumps workflow-compute dependency to include result-preview support.

Reviewed changes

Copilot reviewed 10 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
SPEC.md	Documents the new product-capture step and manifest/runtime step type drift prevention.
README.md	Adds user-facing guidance for using step.compute_product_capture.
plugin.json	Registers step.compute_product_capture in manifest stepTypes.
internal/steps.go	Implements product-capture step, adds state-field URL resolution, and exposes result_preview in outputs.
internal/steps_test.go	Adds manifest-vs-runtime stepTypes assertion and product-capture step tests.
internal/plugin.go	Registers the new step type and wiring in CreateStep.
go.mod	Updates Go/workflow-compute version metadata.
go.sum	Updates sums for the workflow-compute bump.

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[intel352]

Addressed Copilot review: result previews now flatten only an explicit ProductSnapshot allowlist, so reserved/control keys like error are kept inside result_preview only. Step type tests now compare the full expected slice, including step.compute_wait.