[Feature Request]: Redesign and implement CN merge

### Is there an existing issue for the same feature request?

- [x] I have checked the existing issues.

### Is your feature request related to a problem?

PR #28850 removes the legacy CN merge framework because it is outdated and no longer maintained. However, large analytical clusters may still need merge work to use CN compute resources instead of relying only on TN capacity.

### Describe the feature you'd like

Redesign and implement CN merge as a new feature for large analytical workloads.

This issue only records the product need. The architecture and implementation plan will be decided through a separate design process. The removed legacy framework must not be restored or retained as the basis of the new implementation.

### Describe implementation you've considered

To be determined by the design process.

### Documentation, Adoption, Use Case, Migration Strategy

Primary use case: large analytical clusters that need elastic compute capacity for object merge.

### Additional information

- Legacy framework removal: #28850
- Related legacy failures: #28729, #27885