Add callback-driven step rejection/retry for physical constraint handling in stiff3

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stiff3 previously let solout either continue or abort integration, but had no way to mark an accepted step as physically invalid (e.g., positivity violation) and retry with a smaller step. This PR adds a DASSL-like communication path so callers can reject accepted steps and drive the integrator toward physically admissible states.

• Callback contract extension (irtrn)

  • irtrn = 0: keep accepted step (unchanged)
  • irtrn = -1: reject accepted step and retry with reduced step size
  • irtrn < -1: interrupt integration (existing behavior)

• Core integrator retry/backtracking flow

  • On irtrn = -1, the solver now:
    • rolls back to the previous accepted state (x, y, cached RHS/Jacobian context),
    • halves the step size,
    • retries the same step attempt.
  • Adds bounded retry safeguards:
    • per-step maximum physical rejections,
    • minimum-step cutoff tied to integration scale to avoid pathological micro-stepping.

• Documentation and regression coverage

  • README now documents the solout return-code semantics.
  • Added a focused regression test covering:
    • successful one-time reject/retry recovery,
    • repeated rejection path stopping with rollback semantics.

subroutine out(nr, xold, x, y, iha, qa, irtrn)
  ! Reject step if solution violates a physical bound:
  if (any(y < 0.0_wp)) then
    irtrn = -1   ! rollback + halve step + retry
  end if
end subroutine

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