Raise ModelingToolkitBase DelayDiffEq floor

[ChrisRackauckas-Claude]

What changed

Raise the minimum DelayDiffEq version for ModelingToolkitBase from 6 to 6.1. Registered DelayDiffEq 6.0.3 constructs the pre-limiters OrdinaryDiffEqCore.DEOptions signature, which is incompatible with the current OrdinaryDiffEqCore stack; 6.1.0 is the first compatible release. This is a dependency-compatibility fix only: there is no public API change and no deprecation path is required.

Verification

• Exact minimum-version resolver passed:
  [ Info: Successfully resolved minimal versions for lib/ModelingToolkitBase (with extras)
• Failing-before evidence with DelayDiffEq 6.0.3: InitializationSystem Test | 605 passed, 9 errors, 12 broken; process exit RC=1. The errors were DEOptions constructor mismatches.
• Passing-after evidence with DelayDiffEq 6.1.2: InitializationSystem Test | 660 pass, 12 broken; Initial Values Test | 65 pass; Testing ModelingToolkitBase tests passed; process exit RC=0.
• QA: GROUP=QA julia +1.10 --startup-file=no --project=. -e 'using Pkg; Pkg.test()' → QA | 42 42; RC=0.
• TOML parse passed.
• git diff --check passed.

Docs, GPU, and downstream jobs were not run locally; this change does not touch docs or public API.

Please ignore this PR until it has been reviewed by @ChrisRackauckas.