gpl: divergence handling

[gudeh]

This PR includes the following changes:

• Use two snapshots saving, one for divergences and another for routability snapshot.
• Allow reverting when a divergence happens during routability mode.
• Use momentum reset when divergences occur, for default and routability divergences.
• Make routability mode less verbose, demoting multiple messages to debug.

Previously if a divergence happens we would just revert to min HPWL and stop (not reaching 10% overflow target). Now we keep reverting and reseting the momentum up to 3x, allowing us to try and reach the overflow target.

Previously if a divergence happens inside routability we just error and didn't active the "revert if diverge" feature. Now we do revert if a divergence happens during routability. Also, we revert to the routability snapshot resetting the momentum if a divergence happened, if the divergence persists after such revert, we use the default "revert if diverge", reverting to the min HPWL.

This PR addresses issue https://github.com/The-OpenROAD-Project-private/OpenROAD-flow-scripts/issues/1883

[gemini-code-assist]

Code Review

This pull request refactors snapshot management in the Nesterov global placement engine to support multiple snapshot slots (Routability and Diverge), allowing separate recovery paths. It also transitions several verbose log messages to debug prints and refactors congestion tracking to use consistent tile-based overflow statistics. The review feedback highlights critical safety and correctness issues, including potential out-of-bounds accesses when indexing snapshot vectors and target density arrays, a short-circuit evaluation bug in divergence recovery that causes unintended state mutations, and a minor namespace qualification issue.

[gemini-code-assist]

On line 282, a check is performed to see if minRcTargetDensity_ is empty: minRcTargetDensity_.empty() ? 0.0f : minRcTargetDensity_[0]. However, inside the subsequent loop (lines 286-293), minRcTargetDensity_[j] is accessed directly without any bounds check. If minRcTargetDensity_ is indeed empty or has fewer elements than nbVec_.size(), this will result in an out-of-bounds access and potential crash. A bounds check should be added inside the loop, or the loop should be guarded.

Suggested change

	debugPrint(log_,
	GPL,
	"routability",
	1,
	"Target density at minimum routing congestion: {:.4f}{}",
	minRcTargetDensity_[j],
	nbVec_[j]->getGroup()
	debugPrint(log_,
	GPL,
	"routability",
	1,
	"Target density at minimum routing congestion: {:.4f}{}",
	j < minRcTargetDensity_.size() ? minRcTargetDensity_[j] : 0.0f,
	nbVec_[j]->getGroup()

[gemini-code-assist]

Use std::string instead of string to be consistent with the rest of the codebase and to avoid potential compilation errors if std::string is not imported into the global namespace.

Suggested change

	nbVec_[nb_index]->getGroup()
	? " in group " + string(nbVec_[nb_index]->getGroup()->getName())
	: "");
	nbVec_[nb_index]->getGroup()
	? " in group " + std::string(nbVec_[nb_index]->getGroup()->getName())
	: "");

[gudeh]

This should be a much better solution than the closed PR #11505.