dbSta: Adds iteration free delay model to STA

[QuantamHD]

This new delay model removes all iteration from DmpCeffTwoPole series of delay models. This is accomplished via two different methods.

This commit replaces the computationally expensive Newton-Raphson iteration loop in the DMPCeff algorithm with a fully feed-forward, O(1) rational function (Padé approximant).

The new technique pre-characterizes the Pi-model shielding factor (k) offline and evaluates it at runtime using a 18-parameter polynomial optimized for FMA instructions.

To ensure the surrogate model is universal (PDK-agnostic) and strictly respects physical boundaries at extreme asymptotes, the Pi-model physicals are mapped into a dimensionless 3D parameter space:

• x = R_pi / R_d : The resistance ratio. Dictates the primary shielding effect. Squaring this term in the denominator guarantees complete shielding (k=0) as wire resistance approaches infinity.
• y = C_2 / (C_1 + C_2) : The capacitance ratio. Represents the far-end fraction of the total load, strictly bounded between 0.0 and 1.0.
• z = t_r / (R_d * C_tot) : The normalized slew ratio. Maps the driver's unshielded transition time against the intrinsic RC time constant of the network.

The surrogate model evaluates the shielding factor as k = f(x,y,z), allowing effective capacitance to be resolved instantly via: C_eff = C_1 + k * C_2

The resulting C_eff is subsequently passed to a Boost Lambert W (W0) analytical solver to compute the exact timing threshold crossings without any runtime iteration.

Type of Change

• New feature

Impact

OpenSTA Delay Calculator API	Total Runtime (150,000 calls)	Avg Latency / Call	Cumulative Delay Sum
DmpCeffTwoPoleDelayCalc::gateDelay (OpenSTA internal NR loop)	69.41 ms	~462.7 ns	1.458 µs
DmpCeffLambertWDelayCalc::gateDelay (Iteration-free Padé surrogate)	19.34 ms	~128.9 ns	1.458 µs

Verification

• [ x ] I have verified that the local build succeeds (./etc/Build.sh).
• [ x ] I have run the relevant tests and they pass.
• [ x ] My code follows the repository's formatting guidelines.
• [ x ] I have included tests to prevent regressions.
• [ x ] I have signed my commits (DCO).

[gemini-code-assist]

Code Review

This pull request introduces a new delay calculator, DmpCeffLambertWDelayCalc, which utilizes a Lambert W function solver and a bivariate Padé approximation for Ceff shielding calculations. It also adds a training data generator, a Python fitting script, and corresponding unit tests. The review feedback highlights two critical issues in DmpCeffLambertWDelayCalc.cc: first, the real parts of the poles returned by poleResidue must be negated to prevent exponential overflow and NaN values in the mathematical derivations; second, adding 1e-15 to a single-precision float capacitance value will underflow, causing a division-by-zero error when calculating rd.