Convenient constructors for commonly-used survival distributions. Each provides the complete specification (rate, cum_haz_rate, score_fn, and where practical, hess_fn) for optimal performance.
Left-Censoring Note
The analytical score and Hessian functions provided by these constructors
assume event indicators in {0, 1} (right-censored and exact observations).
For left-censored data (delta = -1), these functions are not applicable and
the package automatically falls back to numerical differentiation via
numDeriv::grad and numDeriv::hessian through the log-likelihood,
which handles all censoring types correctly.