search: POCV per-stage sigma holder + LVF variance injection + LVF-library gate (stacks on #383)

[saurav-fermions]

Summary

Adds a parametric statistical-OCV (POCV / LVF) capability in three layered, default-OFF slices. With POCV inactive, timing is byte-identical to upstream.

1. POCV per-stage sigma state holder (Search.hh)

Adds a default-OFF Search::PocvSigma state holder (enabled, per-stage sigma fraction, n_sigma, active()) plus inline pocvSigma()/setPocvSigma() accessors and a pocv_sigma_ member. Read only by an external report-only POCV command; no forward-search code reads it. Parametric POCV variation combines in quadrature (root-sum-square), which does not compose with the additive arrival sum the search propagates, so this holder is intentionally not a propagation hook.

2. LVF propagation-time variance injection (Search.cc, Search.hh)

Injects the synthetic per-stage POCV sigma (k*d_i)^2 into combinational data-path arc delays inside deratedDelayData when PocvSigma is in -propagate mode. The native statistical delay-ops (DelayOpsNormal) then accumulate the variance in quadrature through delaySum during the forward search, reporting arrival ± n_sigma·sqrt(variance). Factors the scalar (mean) data-path derate into deratedDelayDataMean; layers variance on top without changing the mean. Default OFF → zero variance → byte-identical baseline.

3. LVF-lib library-driven POCV gate (Search.hh)

Adds PocvSigma::from_liberty so set_pocv_sigma -from_liberty sources the per-stage delay sigma from real Liberty LVF ocv_sigma_* tables via the native statistical delay calc (GateTableModel::gateDelayPocv under PocvMode::normal) instead of the synthetic global per_stage. In this mode propagateActive() returns false so the synthetic (k*d) injection is suppressed and cannot overwrite the library-derived stdDev. Default from_liberty=false keeps the -propagate/-sigma behavior and a byte-identical flag-off baseline.

⚠️ Depends on #383 (AOCV depth-derate + SI-window)

This is a stacked PR on top of #383. It layers directly on the deratedDelayData hook that #383 introduces (patch 2 splits it into deratedDelayDataMean + variance). GitHub will show the two #383 commits here until that PR merges; after it merges I'll rebase this onto master and only the three POCV commits will remain. Please review/merge #383 first.

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).
• POCV report-only: worst_slack/WNS/TNS byte-identical POCV-on vs off; quadrature direction proven (sqrt-of-N). LVF-propagate: real sqrt(N) de-pessimism; default-OFF byte-identical after propagate+reset. LVF-from-Liberty: sources per-cell/per-arc sigma from Liberty LVF tables; from_liberty=false byte-identical baseline.

Notes

• clang-format intentionally not run on src/sta/* per project convention.
• All three slices are default-off; no behavior change for existing users.

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