grt: implement CUGR net merge mirroring FastRoute path

[sparsh-karna]

Summary

When GlobalRouter::mergeNetsRouting() fires for the CUGR path, this PR enables attempting to stitch the two existing route segments at the former buffer pin position (connectCUGRRouting) rather than ripping up and rerouting the survivor from scratch.

Key changes:

• GlobalRouter::connectCUGRRouting(preserved, removed): Mirrors connectRouting() but uses cugr_->hasAvailableResources() instead of fastroute_->hasAvailableResources() for the capacity check. Reuses the existing geometry helpers findBufferPinPostions, findTopLayerOverPosition, and createConnectionForPositions, which are FastRoute-agnostic.

• GlobalRouter::mergeNetsRouting() (CUGR branch): If stitching succeeds, calls cugr_->mergeNet() to transfer tree ownership and save guides. On failure, falls back to addDirtyNet() so the survivor is rerouted incrementally (matching the previous safe behavior).

• CUGR::mergeNet(preserved, removed): Attaches the removed net's GRTreeNode subtree as a child of the preserved net's routing tree so getRoutes() emits all wire segments. Inserts the removed net into merged_nets_ so removeNet() knows not to decrement GridGraph demand for it (the wires are still physically there, just owned by the preserved net).

• CUGR::removeNet() (merged-net guard): Skips removeTreeUsage() for nets that were transferred via mergeNet, preventing double-decrement of GridGraph congestion numbers.

• CUGR::hasAvailableResources(layer, tile_x, tile_y): A thin wrapper over GridGraph::getEdge().getResource() >= 1.0, equivalent to FastRouteCore::hasAvailableResources().

Type of Change

• New feature

Impact

Improves QoR (Quality of Results) and runtime during incremental routing by preserving existing routing topology during buffer removal ECOs when using CUGR, matching FastRoute's behavior.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass. (All 143 GRT regression tests pass.)
• My code follows the repository's formatting guidelines.
• I have included tests to prevent regressions.
• I have signed my commits (DCO).

Related Issues

None.

[gemini-code-assist]

Code Review

This pull request implements support for merging nets in CUGR rather than always falling back to a full dirty-net reroute. It introduces connectCUGRRouting to stitch two CUGR routes together at the former buffer pin position and perform capacity checks against the CUGR GridGraph, and updates the CUGR class to handle routing tree ownership transfer during net merges. Feedback was provided to add an early return check if either route is empty to prevent a potential crash when finding the top layer over a position.

[gemini-code-assist]

If either net1_route or net2_route is empty, calling findTopLayerOverPosition downstream will fail to find any segments and trigger a program-terminating logger_->error call. Adding an early return check for empty routes prevents this crash and safely falls back to the dirty-net rerouting path.

  GRoute& net1_route = routes_[db_net1];
  GRoute& net2_route = routes_[db_net2];

  if (net1_route.empty() || net2_route.empty()) {
    return false;
  }

[eder-matheus]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the actual CUGR stitch edge

When the incremental session ends, the CUGR path rebuilds routes_ from cugr_->getRoutes() in updateDirtyRoutes(), so the stitch segments appended only in connectCUGRRouting() are discarded. This root-to-root attachment makes getRoutes() synthesize a direct edge between the two tree roots instead of the connection between the former buffer pins; for merged nets whose CUGR roots are not those pins, the final guides/parasitics describe the wrong topology. The CUGR tree needs to encode the actual connection geometry before taking ownership of the removed tree.

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