Declare custom-integrator construction and stepping hooks public (#4081)

Declares fix_dt_at_bounds!, handle_tstop!, initialize_tstops,
initialize_saveat and initialize_d_discontinuities public in
OrdinaryDiffEqCore. These are the hooks packages use to build their own
integrator on top of the OrdinaryDiffEqCore machinery; DelayDiffEq and
OrdinaryDiffEqOperatorSplitting both call them and add methods to them.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

lib/OrdinaryDiffEqCore/Project.toml

@@ -1,7 +1,7 @@
 name = "OrdinaryDiffEqCore"
 uuid = "bbf590c4-e513-4bbe-9b18-05decba2e5d8"
 authors = ["ParamThakkar123 <paramthakkar864@gmail.com>"]
-version = "4.12.1"
+version = "4.13.0"
 
 [deps]
 ADTypes = "47edcb42-4c32-4615-8424-f2b9edc5f35b"

lib/OrdinaryDiffEqCore/src/OrdinaryDiffEqCore.jl

@@ -488,6 +488,12 @@ include("precompilation_setup.jl")
             # of these, so they are a required solver-author extension point.
             :StochasticDiffEqCache, :StochasticDiffEqConstantCache,
             :StochasticDiffEqMutableCache, :trivial_limiter!,
+            # Round 3: custom-integrator construction and stepping hooks. Packages that
+            # build their own integrator on top of the OrdinaryDiffEqCore machinery
+            # (DelayDiffEq, OrdinaryDiffEqOperatorSplitting) both call these and add
+            # methods to them for their own integrator types.
+            :fix_dt_at_bounds!, :handle_tstop!,
+            :initialize_tstops, :initialize_saveat, :initialize_d_discontinuities,
         )
     )
 end