Drop deprecated StochasticDelayDiffEq dependency

[ChrisRackauckas-Claude]

Motivation

StochasticDelayDiffEq.jl is deprecated and will not receive a v7-compatible release. From OrdinaryDiffEq.jl NEWS.md:

StochasticDelayDiffEq deprecated

StochasticDelayDiffEq.jl is deprecated. Use DelayDiffEq.jl directly — it has supported SDDE problems for some time, and the separate StochasticDelayDiffEq wrapper is no longer being maintained. It will not receive a v7-compatible release.

Migration: replace using StochasticDelayDiffEq with using DelayDiffEq (plus using StochasticDiffEq if you were relying on the SDE algorithm re-exports). MethodOfSteps(alg) and the SDDEProblem constructor continue to work from DelayDiffEq / SciMLBase respectively.

Keeping SDDE as a dependency forces MTK to cap itself out of the v7 ecosystem. This PR removes it, unblocking downstream work (BaseModelica#120, MTKStdlib#445).

Changes

Project.toml (top-level)

Before:

# [compat]
StochasticDelayDiffEq = "1.11"

# [extras]
StochasticDelayDiffEq = "29a0d76e-afc8-11e9-03a4-eda52ae4b960"

# [targets]
test = [..., "StochasticDelayDiffEq", ...]

After: all three entries removed. DelayDiffEq and StochasticDiffEq were already present.

lib/ModelingToolkitBase/Project.toml

Same removal — StochasticDelayDiffEq dropped from [compat], [extras], and [targets].

lib/ModelingToolkitBase/test/initializationsystem.jl

Before:

using StochasticDiffEq, DelayDiffEq, JumpProcesses
# FIXME: Disabled due to failing precompilation
# using StochasticDelayDiffEq

After:

using StochasticDiffEq, DelayDiffEq, JumpProcesses

The commented-out import was already dead code — DelayDiffEq (already on line 2) covers all SDDE solver needs.

SDDEProblem and SDDEFunction references elsewhere in the codebase are unaffected — they come from SciMLBase, not StochasticDelayDiffEq.

Local verification

Re-resolve of lib/ModelingToolkitBase picks up the full v7 line:

DiffEqBase  => 7.0.0
SciMLBase   => 3.6.0
OrdinaryDiffEqCore => 4.0.0

No StochasticDelayDiffEq entry in the resolved manifest.

Test plan

• CI green on Core (no code logic changed, only manifest/test imports)
• Re-resolve in a clean environment confirms v7 ecosystem resolves