Move Enzyme tests to separate test group

[ChrisRackauckas-Claude]

Summary

• Created test/enzyme/ directory with its own Project.toml for Enzyme dependencies
• Moved all Enzyme-specific tests to test/enzyme/runtests.jl
• Removed Enzyme from main test dependencies in Project.toml
• Added separate CI job that only runs Enzyme tests on Julia LTS version

This follows the OrdinaryDiffEq.jl pattern for test groups with separate Project.toml files, which:

1. Isolates Enzyme dependencies from the main test suite
2. Avoids Enzyme compatibility issues with pre-release Julia versions (EnzymeRuntimeActivityError on Julia 1.12+)
3. Keeps the main test suite running on all Julia versions (lts, 1, pre)

Test plan

• CI passes for main tests on all Julia versions (lts, 1, pre)
• CI passes for Enzyme tests on Julia LTS
• Verify Enzyme tests cover the same functionality as before

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

CI Status Update

All relevant tests are now passing:

• ✅ Enzyme Tests (lts) - PASS - The fix for the BackTracking name conflict worked
• ✅ Tests (1) - PASS
• ✅ Tests (lts) - PASS
• ✅ runic - PASS
• ✅ build - PASS
• ✅ Spell Check - PASS
• ✅ test (alldeps, 1.10) - PASS

The only failure is Tests (pre) which is a pre-existing Julia 1.13-alpha compatibility issue with ReTestItems:

MethodError: no method matching setindex!(::Base.ScopedValues.ScopedValue{Bool}, ::Bool)

This failure also occurs on the main branch (see workflow runs from Jan 4-8, 2026) and is not related to these changes.