grt: add CUGR global_route_debug -st visualization support

[hansb21]

Summary

This PR adds global_route_debug -st visualization support for
CUGR, aligned with the existing FastRoute debug flow in GUI.

Main updates:

• Enabled global_route_debug -st when running with global_route -use_cugr.
• Implemented CUGR debug infrastructure
• Added the debug visualization trigger during CUGR Stage 1
  (pattern routing).

Type of Change

• New feature

Impact

Users can now visualize the Stage-1 Steiner tree for CUGR in GUI
using:

1. global_route_debug -net <net_name> -st
2. global_route -use_cugr

FastRoute debug behavior remains available.

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).

Visual Evidence

Visual Evidence

FastRoute (global_route_debug -net "net1707" -st + global_route)

CUGR (global_route_debug -net "net1707" -st + global_route -use_cugr)

Related Issues

N/A

[github-actions]

Welcome to OpenROAD! Thanks for opening your first PR.
Before we review:

• Contribution Guide: https://openroad.readthedocs.io/en/latest/contrib/contributing.html
• Build Instructions: https://openroad.readthedocs.io/en/latest/contrib/BuildWithCMake.html

Please ensure:

• CI passes
• Code is properly formatted
• Tests are included where applicable
  A maintainer will review shortly!

[gemini-code-assist]

Code Review

This pull request introduces comprehensive debugging and visualization support for the CUGR global router, bringing it to parity with the FastRoute debug interface. Changes include new debug initialization and retrieval methods in the GlobalRouter class, updates to the SWIG interface files, and the implementation of Steiner tree visualization routines within the CUGR engine. The review feedback identifies several critical improvements: addressing potential dangling pointers by clearing the renderer after use, replacing hardcoded magic numbers with design-specific values for net properties, and refactoring duplicated FrNet initialization logic into a shared helper function.