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.

Impact

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

Testing

• Passed the OpenROAD dmp_ceff_dcalc_load_propagation Bazel regression with fresh placement estimation for Elmore, TwoPole, and Lambert-W.
• Passed OpenROAD build, Tcl lint/format, dbSta lint, clang-tidy, and direct CLI QA.

Related

• dbSta: Fix zero slew and net delay issue in dmp_ceff_two_pole and dmp_ceff_lambert_w delay calculators OpenROAD#11229

[gemini-code-assist]

Code Review

This pull request overrides the reduceParasitic method in DmpCeffTwoPoleDelayCalc to reduce the parasitic network using reduceToPiPoleResidue2. Feedback was provided to add a defensive null check for the parasitics pointer returned by scene->parasitics(min_max) to prevent a potential null pointer dereference crash.

[gemini-code-assist]

The scene->parasitics(min_max) call can return nullptr if parasitics are not loaded or available. Dereferencing parasitics without a null check will cause a crash. Please add a defensive null check before calling reduceToPiPoleResidue2.

  Parasitics *parasitics = scene->parasitics(min_max);
  if (parasitics == nullptr)
    return nullptr;
  return parasitics->reduceToPiPoleResidue2(
      parasitic_network, drvr_pin, rf, scene, min_max);

[jhkim-pii]

Opened in the wrong repository; closing this PR.