psm: report the average IR drop under the drop__average metric

[shivarammysore]

Fixes #11096, with both changes @gadfort asked for.

design_powergrid__drop__average passed results.avg_voltage, so on a power net it reported the supply voltage instead of the drop. avg_ir_drop was already computed in getSolution() and used correctly in the text report two lines earlier — it just never reached the metric.

It stayed hidden because on a ground net net_voltage == 0, so getSolution() sets avg_ir_drop = avg_voltage and the metric is accidentally right. Every run reports both a power and a ground net, so half the numbers always looked correct. On a 1.8 V power net the metric read 1.79999 where the average drop was 6.41e-06.

Changes

• design_powergrid__drop__average now carries results.avg_ir_drop.
• Adds design_powergrid__voltage__average carrying results.avg_voltage, so the value the old metric held is still published and named for what it is — and sits alongside the existing design_powergrid__voltage__worst.

Notes

• No test goldens or docs reference these metric names, so nothing else needed updating — the two names are used only here.
• The text report is unchanged; it was already correct.
• Consumers keying on drop__average for power nets will see the value change (to the drop it was always named for); ground-net values are unaffected.

[gemini-code-assist]

Code Review

This pull request updates the metric logging in IRSolver::report by correcting the metric key for average voltage to design_powergrid__voltage__average and adding a new metric log for average IR drop with the key design_powergrid__drop__average. There are no review comments, so I have no feedback to provide.

[github-actions]

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[gadfort]

LGTM. Thanks.