Ignore until reviewed by @ChrisRackauckas.

Summary

• add is_disco_step and disco_checkpoint to DDEIntegrator, matching the state required by OrdinaryDiffEqCore's shared autonomous-discontinuity controller
• initialize the state from the runtime time value and clear it in reinit!
• require OrdinaryDiffEqCore 4.12, the first release line that contains the corresponding DISCO controller behavior
• cover the public init / solve! / reinit! path with discontinuity detection enabled, plus Unitful DDEs

Root cause

OrdinaryDiffEqCore's shared DISCO/controller code assumes that each compatible integrator carries is_disco_step and disco_checkpoint. ODEIntegrator gained that state as the DISCO implementation evolved, but DDEIntegrator did not. An owner-clean current dependency stack therefore reaches DDEIntegrator and throws type DDEIntegrator has no field is_disco_step; the same shared path needs disco_checkpoint once that first field exists.

A fixed-dependency parent/child check and git bisect identify 408a7a30fe0989a55b446240a18fb2a8c5c30993 (disco optimizations; parent c538565b25ff19e661e5bf788fe1953a154d64e9) as the first commit that made the shared DISCO path incompatible with DDEIntegrator. With the same historical dependency manifest, a QNDF DDE reaches Success at the parent and the child fails on the earlier shared-controller field curr_discontinuity. The evolved current-master path first exposes the missing is_disco_step field instead.

disco_checkpoint is initialized with zero(t0), preserving units and other runtime time metadata. reinit! clears both values so a reused integrator cannot carry transient DISCO state from its previous solve.

Version boundary

General currently ends at OrdinaryDiffEqCore 4.11.0, while the monorepo's next version is 4.12.0. DelayDiffEq must therefore require OrdinaryDiffEqCore = "4.12"; allowing the previous broad "4" range could pair the new DelayDiffEq integrator assumptions with an older Core controller.

The branch is rebased onto current master, including #4069's time-value checkpoint initialization. #4066 is not part of this branch. Because current monorepo source requires SciMLBase 3.40 while General still provides 3.39.1, the final package test used an ignored manifest with a clean local SciMLBase 3.40.1 checkout and showed OrdinaryDiffEqCore 4.12.0 in the test environment.

Validation

• clean current-source QNDF DDE reproduction: fails before this patch with missing is_disco_step
• final commit e13f39aa3b, official DelayDiffEq GROUP=Interface: package test passed; discontinuities 48/48, mass matrix 7/7, Unitful 21/21, and multi-algorithm 28/28
• Runic passes on all five changed Julia files
• git diff --check passes

Whole-tree Runic still reports only the three pre-existing DISCO formatting files handled by #4064. The earlier GROUP=QA run reached an independent existing ExplicitImports public-ownership cluster; none of its names or files are touched here.