ipw 1.3.1

• Core Refactoring (ipwtm, ipwpoint, ipwplot, tstartfun):
  • Improved defensive input validation, parameter checking, and error messages across all core functions.
  • Modernized formula construction (as.formula()) to eliminate fragile eval(parse(text = ...)) string manipulations.
  • Fixed variable symbol evaluation issues when passing variables as function arguments rather than raw names.
  • Modularized ipwtm into dedicated family modeling engines (binomial, survival, multinomial, ordinal, gaussian).
  • Replaced manual survival hazard step-differencing with continuous baseline hazard interpolation using survival::basehaz and stats::approx.
  • Refactored tstartfun to use safe column evaluation and direct vector lag logic (c(-1, t[-length(t)])).
• Test Suite Expansion:
  • Upgraded testthat suite to 3rd edition specs, expanding coverage for multinomial, ordinal, gaussian, unstabilized models, weight truncation, and invalid parameter rejection.