Add reg2reg _ws in reports.json files and regression checks

[oharboe]

Description

In designs where a macro (e.g., mock-array) is constrained only with set_max_delay to drive internal optimization, input/output constraints are intentionally overconstrained and do not reflect meaningful timing violations. As such, WNS values for in2reg, reg2out, and in2out paths are not relevant for performance regression tracking. The only meaningful metric in this context is reg2reg WNS, which reflects the true critical timing within the macro.

It would be very useful if OpenROAD could include a separate wns_reg2reg (or similar) field in report.json, computed from paths where both endpoints are registers. This would allow for more accurate CI-based performance regression tracking without being skewed by intentionally pessimistic IO constraints.

Suggested Solution

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Additional Context

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