GRT: fix resources update for back-to-back jumpers during repair antennas

[jfgava]

Summary

updateRouteGridsLayer now builds a new grids vector instead of modifying layers in place. When a promoted region begins or ends adjacent to an un-promoted point, it inserts an extra GPoint3D at the boundary position keeping the old layer, creating an explicit via-transition pair in the sttree.

Why this fixes the underflow:

Before the fix, after two adjacent jumpers (x=105→107 on l3→l5, then x=108→110 on l3→l5):

(107,l5) → (108,l5) ← same layer, treated as h-edge by releaseNetResources
→ decrements h_edge[5][148][107] which was never incremented → underflow

After the fix:

(107,l5) → (107,l3) → (108,l3) → (108,l5)
(107,l5)→(107,l3): layer change → skipped as via
(107,l3)→(108,l3): h-edge on l3 at x=107 → decremented correctly (was incremented when the net was originally routed on l3)
(108,l3)→(108,l5): layer change → skipped as via

The same via-transition is also inserted at the outer endpoints of each jumper (e.g. x=105 and x=110), which fixes a pre-existing minor leak on the approach/departure edges at those boundaries.

Type of Change

• Bug fix

Impact

Properly update fast route resources after inserting back-to-back jumpers.

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

Underflow issue

[gemini-code-assist]

Code Review

This pull request addresses an adjacent-jumper sttree underflow bug in global routing by modifying updateRouteGridsLayer in FastRoute.cpp to insert old-layer via-transition points at the boundaries of promoted regions, preventing back-to-back jumpers from merging. Additionally, it optimizes RepairAntennas.cpp by using emplace_back instead of push_back, adds a regression test for the bug, and updates the corresponding test metrics. The review feedback suggests replacing C-style casts with static_cast<int16_t> in the newly added code in FastRoute.cpp to align with modern C++ standards and improve type safety.