grt/cugr: implement resistance-aware stage and resistance costs during congestion handling

[eder-matheus]

Summary

Adds resistance-aware global routing to the CUGR engine. With -resistance_aware
and a non-zero -critical_nets_percentage, timing-critical nets are biased onto
lower-resistance upper-metal layers to improve post-route timing.

Mechanism:

• Layer sheet/via resistance plumbed from the tech DB into the CUGR cost model.
• Critical-net selection mirroring FastRoute's updateSlacks (clock/NDR always;
  short/single-pin/positive-slack excluded; rest ranked by a multi-factor
  slack/resistance/fanout/length score). The set accumulates across passes.
• A new "Stage 2" resistance-aware re-route that, after the neutral first pattern
  route, re-routes the critical set on real per-net tree resistance.
• A wire/via resistance cost term in PatternRoute, applied only to res-aware nets.
• A congestion resource gate that biases full edges toward climbing vias.
• Optional debug dumps via -debug_level GRT resAware <1|2>.

Type of Change

• New feature

Impact

• New opt-in routing behavior; inert unless -resistance_aware is set with a
  non-zero -critical_nets_percentage.
• Default critical_nets_percentage for CUGR changed 0 -> 10.
• CUGR guides are no longer flagged congested (DRT resolves them), removing
  spurious congestion warnings on the CUGR path.
• New warning GRT-0309 when timing is unavailable and the critical-net
  percentage is forced to 0 (matches the FastRoute path).
• Several existing CUGR test goldens shift due to the default change, stage
  renumbering, and the new logs.

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

None.

[gemini-code-assist]

Code Review

This pull request introduces resistance-aware global routing to the CUGR engine, enabling the biasing of critical nets onto lower-resistance upper metal layers to improve timing. It adds support for calculating net resistance, scoring nets based on slack, resistance, fanout, and length, and re-routing critical nets in a new stage. Additionally, a congestion gate penalty is introduced to bias vias upward when edges are full. Feedback on these changes highlights three critical issues: a logic bug in markResAwareNets that incorrectly skips short or positive-slack clock and NDR nets, a missing null check in the res_aware_order sorting comparator that could lead to segmentation faults, and a missing null check on sta_ in initCUGR that could cause crashes when STA is uninitialized.

[eder-matheus]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 27cd0dc034

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[eder-matheus]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: da1dbdca96

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