docs(rust): add cloudfetch pipeline redesign spec and sprint plan (#300)

## Summary

- Add CloudFetch pipeline redesign spec with channel-based architecture
design
- Add sprint plan with task breakdown and DoD checklist
- Update spec with channel design details (bounded result channel,
unbounded download channel)

This is a design-only PR with no code changes.

## Stacked PRs
- [**stack/task-0-cloudfetch-design**](this PR) — design docs
-
[stack/task-1-config-types-and-pipeline-types](#283)
— impl: config + pipeline types
-
[stack/task-2-scheduler-workers-consumer-rewrite](#284)
— impl: scheduler, workers, consumer

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Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: eric-wang-1990