CTS: fix one sink on cluster

[openroad-ci]

Summary

The sink clustering algorithm sometimes generates clusters with a single sink, which negatively impacts the skew.
This modification adds a function that attempts to repair these single-sink clusters by merging them into other clusters or adding sinks that are close to them.
If repair is not possible, a buffer will then be added to these types of clusters as well.

Type of Change

• Breaking change

Impact

This modification is expected to result in a better clock tree construction, as it resolves the issue of clusters with only one sink.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have signed my commits (DCO).

Related Issues

Fix issue #9070

[gemini-code-assist]

Code Review

This pull request introduces a new method repairClusteringSolution in SinkClustering to repair single-sink clusters by either merging them into nearby clusters or moving nearby sinks. It also integrates this repair step into findBestMatching and updates the corresponding test expectations. The review feedback highlights a critical signed/unsigned comparison bug in the repair loop where decrementing the signed index c to -1 and comparing it against the unsigned cluster_num causes premature loop termination. Additionally, the reviewer suggests declaring loop variables consistently to avoid warnings and refactoring the inner loop to use index-based iteration to prevent unnecessary copies of Point<double>.

[luis201420]

These modifications require updating the metrics (PR The-OpenROAD-Project/OpenROAD-flow-scripts#4262)

[maliberty]

You also need to update metrics in the test/ directory in OR