search/parasitics: AOCV depth-based derate hook + SI-window coupling-cap setter

[saurav-fermions]

Summary

Two flag-gated, default-no-op OpenSTA hooks that let an external driver (OpenROAD dbSta/rcx) refine timing without changing stock OpenSTA behavior. With no hook installed, timing is byte-identical to upstream.

1. AOCV depth-based derate (search/Search.{hh,cc})

Adds an abstract AocvDepthDerate hook and Search::deratedDelayData so the OCV derate applied to data-path combinational arcs during forward arrival propagation can be selected by the path's accumulated combinational depth instead of the flat SDC factor.

• When no hook is installed (the default), deratedDelayData forwards verbatim to deratedDelay → byte-identical to upstream.
• Only data-path combinational cell arcs are depth-derated; clock/reg/latch/check arcs keep the flat derate.
• Depth is the count of combinational arcs on the live from_path prev chain + 1.
• A lateActive()/earlyActive() per-side gate prevents a late-only table from silently zeroing a user's set_timing_derate -early on the min path.

2. SI-window per-coupling-capacitor value setter (parasitics/*)

Adds Parasitics::setCapacitorValue (default no-op) plus a ConcreteParasitics override / ConcreteParasiticDevice::setValue, so a timing-window-aware coupling derate can scale individual coupling caps. Only invoked from the external window path; stock OpenSTA behavior is unchanged.

Testing

• Full OpenSTA ctest suite: 6083/6084 pass. The single failure (tcl.power.vcd_detailed) is a pre-existing last-digit floating-point rounding diff in the power module that fails identically on unmodified master (unrelated to this change).
• Derate / CRPR / multicorner / OCV-derate regressions all pass; timing is byte-identical with no hook installed.

Notes

• clang-format intentionally not run on src/sta/* per project convention.
• Both hooks are additive and default-off; no behavior change for existing users.

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[gemini-code-assist]

Code Review

This pull request introduces support for setting individual coupling capacitor values to enable timing-window aware coupling derating, and adds an abstract hook (AocvDepthDerate) to support AOCV-style depth-dependent OCV derating during forward arrival propagation. The review feedback suggests adding a defensive null check for the capacitor pointer in ConcreteParasitics::setCapacitorValue to prevent potential null pointer dereferences. Additionally, it points out that traversing the path prefix chain in Search::aocvDataDepth results in O(N^2) complexity, which could become a performance bottleneck for deep combinational paths, and suggests caching or documenting this behavior.

[gemini-code-assist]

To prevent potential null pointer dereferences and crashes, it is highly recommended to add a defensive null check for the capacitor pointer before casting and dereferencing it.

void
ConcreteParasitics::setCapacitorValue(ParasiticCapacitor *capacitor,
                                      float value)
{
  if (capacitor == nullptr)
    return;
  ConcreteParasiticCapacitor *ccapacitor =
    static_cast<ConcreteParasiticCapacitor*>(capacitor);
  ccapacitor->setValue(value);
}

[gemini-code-assist]

Traversing the path prefix chain backwards at every combinational arc propagation step results in $O(N^2)$ complexity with respect to path depth $N$. While this is only executed when the AOCV hook is active, it could become a performance bottleneck for deep combinational paths. Consider if there is a way to cache or store the depth, or at least document this scaling behavior.