pad: avoid segfault when pad placement is not possible

[gadfort]

Summary

Add check for non-convergence / non-progress and error out. And fix out of bound index

Type of Change

• Bug fix

Impact

Avoids a segfault when entire row is blocked and exit early when placement is stuck.

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).

[gemini-code-assist]

Code Review

This pull request fixes an off-by-one boundary check in pad spreading and adds convergence detection to identify when pad positions stop changing across iterations, logging an error and placing instances for debugging if legalization fails. A new test case, 'place_pads_uniform_fullyblocked', is also introduced. Feedback is provided to optimize the convergence check in 'PadPlacer.cpp' by using a 'std::vector' instead of creating and comparing 'odb::PtrMap's, which avoids significant allocation and lookup overhead in each iteration.