Fix downgrade CI: split the AD test group out of the downgrade lane

[ChrisRackauckas-Claude]

Problem

The Downgrade (Core) job was red. The root cause is that the AD test extras (Enzyme, EnzymeCore, Mooncake) were part of the root test target ([targets].test), so the downgrade resolver had to place them in the minimum-version test env.

julia-downgrade-compat (Resolver.jl) minimizes Julia to the floor of its --julia range; on lts that admits 1.10.0. But every Mooncake 0.5.x requires julia >= 1.10.8, and the Enzyme 0.13 + EnzymeCore 0.8 + Mooncake 0.5 minimum-version set has no solution on the 1.10 LTS stdlib line, so the merged test env is Unsatisfiable. This is the Resolver.jl <-> Mooncake wall (StefanKarpinski/Resolver.jl#24). The AD deps should not gate the downgrade lane.

Fix: split the AD group out of downgrade (instead of pinning Julia)

• Move the AD test deps into their own per-group sub-environments: test/Enzyme/Project.toml and test/Mooncake/Project.toml, each with [sources] FunctionWrappersWrappers = {path = "../.."}. Drop Enzyme/EnzymeCore/Mooncake from the root [extras]/[targets].test. (They stay as [weakdeps] + [compat] for the extensions.)
• test/runtests.jl: the Enzyme/Mooncake groups now use SciMLTesting's per-group env = ... form (the same mechanism the nopre group already uses), so each runs in its own env.
• test/test_groups.toml: add Enzyme and Mooncake groups (["lts", "1"]). They were declared in runtests.jl but had no matrix entry, so they were not running in CI at all — now they do.
• The Downgrade job runs GROUP=Core, which resolves only the root test target — now AD-free — so it is satisfiable on lts (Julia 1.10) again.

Reverted #59 accommodations (they masked the AD-resolution problem)

• .github/workflows/Downgrade.yml: julia-version back to "lts" (no longer pinned to 1.11).
• PrecompileTools compat floor back to "1": the 1.0.0 GPUCompiler precompile failure only arose because Enzyme pulled GPUCompiler into the downgrade env; with AD out of that env it no longer applies.

Kept — genuine minimum-version bugs, independent of AD (verified on Julia 1.10)

• TruncatedStacktraces = "1.1": src uses @truncate_stacktrace, which does not exist before v1.1.0 (1.0.0 is a 25-line stub).
• FunctionWrappers = "1.1.2": 1.0.0 / 1.1.0 / 1.1.1 fail to build the FunctionWrapper cfunction (Module IR does not contain specified entry function) on Julia 1.10; 1.1.2 is the lowest that builds and runs. This is exercised by the package's own PrecompileTools @compile_workload, so it is a real floor regardless of the AD split. (This vindicates the earlier FunctionWrappers floor raise — the floor is real; only the exact version was off, 1.1.2 vs 1.1.3.)

Local verification (Julia 1.10 / lts — the actual downgrade leg)

Ran the real julia-actions/julia-downgrade-compat@v2 downgrade.jl (mode deps, projects ., effective skip = Julia stdlibs):

• Resolution: exit 0, Successfully resolved minimal versions for . (with extras).
• Downgraded manifest contains no Enzyme / Mooncake / GPUCompiler. Floors: FunctionWrappers 1.1.2, PrecompileTools 1.0.0, TruncatedStacktraces 1.1.0, SafeTestsets 0.1.0, SciMLTesting 1.0.0.
• Pkg.test GROUP=Core with allow_reresolve=false, force_latest_compatible_version=false (the julia-runtest CI settings): precompiles cleanly and passes — FunctionWrappersWrappers.jl 48/48, BigFloat + UnionAll 7/7.

AD groups still run via their sub-envs (Julia 1.11): GROUP=Enzyme 65/65, GROUP=Mooncake 13/13, with FunctionWrappersWrappersEnzymeExt / FunctionWrappersWrappersMooncakeExt precompiling in-group. The grouped-tests matrix script now emits 9 jobs (Core lts/1/pre, Enzyme lts/1, Mooncake lts/1, nopre lts/1).

Please ignore until reviewed by @ChrisRackauckas