Add DASSL-style ires backtracking with physical rejection retries (idid = -4)

[Copilot]

This change introduces DASSL-like callback semantics so fun can reject physically inadmissible trial steps without hard-stopping integration. ires = -1 now triggers step rollback + halving retry, while ires < -1 remains an interrupt path (idid = -2).

• Solver contract and status model

  • Clarified rhs_sub callback contract in code/docs:
    • ires = 0 → normal
    • ires = -1 → reject trial step and retry with smaller step
    • ires < -1 → interrupt integration
  • Added solver exit code idid = -4 for rejection-limit termination (too many physical rejections).

• Core integration flow (stiff3_core)

  • Added per-outer-step physical rejection counter (nphys) and retry loop.
  • Applied rejection handling at every RHS evaluation point, including accepted-step endpoint RHS evaluation.
  • On ires = -1, solver now restores pre-attempt state (y, f, df), halves h, resets icon, increments rejection stats, and retries.
  • If nphys > 10, solver exits with idid = -4 using restored safe state.

• Stage solver propagation (sirk3)

  • Ensured internal RHS call is ires-aware (ires=0 before call).
  • Returns early on any nonzero ires so caller (stiff3_core) applies -1 retry vs <-1 interrupt semantics.

• Tests

  • Updated existing interrupt test behavior where ires=-1 had been used as a hard interrupt signal (now uses <-1 for interrupt).
  • Added ode_phys_reject coverage for:
    • recovery after transient ires=-1 rejections (idid=0)
    • rejection-limit termination with persistent ires=-1 (idid=-4)
  • Registered new test target in test CMake list.

• Documentation

  • Updated README callback semantics and exit-flag table to include the new ires behavior and idid=-4.

subroutine fun(n, y, f, ires)
  integer, intent(in) :: n
  real(wp), intent(in) :: y(n)
  real(wp), intent(inout) :: f(n)
  integer, intent(inout) :: ires

  f(1) = -y(1)

  ! Ask solver to reject and backtrack this trial step
  if (y(1) < 0.0_wp) ires = -1

  ! Hard interrupt remains available via ires < -1
  ! if (fatal_condition) ires = -2
end subroutine