drt: restore full routing-layer grid

[naveenvenk17]

Fixes #10873.

The failure bisects to 0fe62b1, which stopped adding routing layers above TOP_ROUTING_LAYER to FlexGridGraph. That loop builds the track maps used for maze coordinates and adjacent-layer via decisions as well as the z dimension, so trimming it changes routing topology rather than only removing unused storage.

Restore the previous full routing-layer grid and remove the fixed-object guards that were only needed by the trimmed z range. This gives DRT the same topology as before the regression; the memory optimization can be revisited once it preserves those routing invariants.

I ran the issue's supplied tiny_rocket/IHP130 replay on the GCP VM:

• current master reproduced both DRT-0255 errors exactly at 50% and 70%, including the reported net, coordinates, and route boxes
• this rollback completed iteration 0 without either path failure, completed iteration 1, and continued into iteration 2 until the 20-minute replay cap
• current master used a 5-layer maze grid and peaked around 1.4 GB before aborting
• the restored grid includes the upper technology layers and peaked around 1.9 GB during later optimization, which is the expected tradeoff for reverting the optimization

Repository checks:

• all 26 //src/drt/test:all targets passed
• Bazel --config=lint build for //src/drt:drt passed
• stamped //:openroad build passed
• clang-format --dry-run --Werror
• git diff --check

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[gemini-code-assist]

Code Review

This pull request removes the logic that limits grid graph layers and fixed-shape costing to the TOP_ROUTING_LAYER threshold. Specifically, it removes the layer-skipping checks based on the grid graph's minimum/maximum layer numbers and the top routing layer configuration in both FlexDR_init.cpp and FlexGridGraph.cpp. There are no review comments, so I have no feedback to provide.

[maliberty]

@mguthaus FYI

[mguthaus]

Sounds like it's best to keep it removed. I'm not an expert in the DRT module and was just going for an easy reduction but obviously this isn't that.

[osamahammad21]

The regression is narrower than the grid needing the full stack. The top routing layer gets its wrong-way edges from the tracks of the layer above it, and those track coords also feed the maze x/y coordinate union. So trimming the upper layers out of initTracks deleted all wrong-way edges on the top routing layer plus some nodes below, which is what breaks the maze search here.

None of that needs grid z-planes above the top routing layer, and that's where the memory win comes from. So we can keep collecting tracks one layer above TOP_ROUTING_LAYER while still trimming the z dimension.