pdn: check via enclosure and try again when merging changes width

[gadfort]

Summary

Closes #11129
Iteratively resolves the stacked vias to ensure they are re-evaluated on changes.

Type of Change

• Bug fix

Impact

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 included tests to prevent regressions.
• I have signed my commits (DCO).

Related Issues

#11129

[gemini-code-assist]

Code Review

This pull request updates the via generation logic in the PDN module to correctly handle width-conditioned rules on shared routing layers in a via stack by iteratively rebuilding the stack based on the merged metal widths. It also transitions via storage to use std::unique_ptr for safer memory management. The review feedback suggests several improvements: updating getLowerHeight and getUpperHeight to handle horizontal shared routing layers, resolving signed/unsigned comparison warnings by using size_t in loops, adding a maximum iteration limit to the rebuilding loop to prevent infinite loops, and adding default member initializers to the Constraint struct to avoid uninitialized variables.