dcalc: Restore TwoPole parasitic reduction

[jhkim-pii]

Summary

• Restore calculator-specific PiPoleResidue2 reduction for dmp_ceff_two_pole.

Problem

• The parasitics API migration removed the TwoPole reduced-type selector without adding the equivalent reduceParasitic override.
• Eager reduction therefore inherited LumpedCapDelayCalc and produced PiElmore, leaving TwoPole without pole/residue data after the detailed network was deleted.

Solution

• Override DmpCeffTwoPoleDelayCalc::reduceParasitic to call reduceToPiPoleResidue2.
• Keep the inherited net-level reduction overload visible for generic callers.
• Add a regression that reduces a detailed SPEF network and verifies the result is PiPoleResidue.

Impact

• TwoPole consumers receive the parasitic representation required by their load-delay model.
• DMP Ceff Elmore reduction remains unchanged.

Related

• https://github.com/The-OpenROAD-Project-private/OpenROAD/pull/3699

[gemini-code-assist]

Code Review

This pull request implements the reduceParasitic method in DmpCeffTwoPoleDelayCalc to reduce a parasitic network to a Pi-pole residue, and adds a corresponding unit test to verify this functionality. The feedback suggests adding a null check for the parasitics pointer retrieved from the scene to prevent a potential null pointer dereference before calling reduceToPiPoleResidue2.