Require DiffEqFlux 4

[ChrisRackauckas-Claude]

Summary

Ignore this PR until it has been reviewed by @ChrisRackauckas.

• require DiffEqFlux 4
• drop the obsolete DiffEqFlux 2 and 3 compatibility ranges from the downgrade graph

The patched downgrade resolver selects DiffEqFlux 4.0.0 exactly. This is a focused compatibility cleanup; it does not change Optimization source code or tests.

Local validation

• Patched-action locked Core environment on Julia 1.10: selected DiffEqFlux 4.0.0. The run reached the existing Optim 1 interface incompatibility (7 passed, 3 errors).
• Clean master with the same locked dependency graph: reproduced the same 7-passed/3-error Optim 1 failures. That independent base-branch incompatibility is being investigated separately.
• Current-resolution GROUP=Core on Julia 1.12: passed (Mini batching 2 passed/1 pre-existing broken; Interface 14/14; Sense 18/18).
• Current-resolution GROUP=QA on Julia 1.12: passed (18 passed/1 pre-existing broken).
• Runic.main([\"--check\", \".\"]): passed.
• git diff --check: passed.

The exact-floor failure is therefore identical on clean master, while this branch independently validates the intended DiffEqFlux floor and passes the current test and QA groups.

[ChrisRackauckas-Claude]

Investigation scratchpad: the patched downgrade resolver selected DiffEqFlux 4.0.0 after removing the 2/3 ranges. I compared that locked graph against an untouched clean-master checkout and reproduced the same Optim 1 interface errors in both (7 passed, 3 errors), isolating those failures from this compat edit. The branch then passed current Core, current QA, Runic, and diff checks locally.