Split AD tests into a dedicated off-downgrade group (Mooncake 0.5 needs >= 1.10.8)

[ChrisRackauckas-Claude]

Problem

The Downgrade (Core) job is red on master with:

[ Info: Running resolver on merged project (extras) for . with --min=@deps
ERROR: LoadError: Unsatisfiable

Root cause

The SciML downgrade workflow (SciML/.github downgrade.yml) builds and tests with project: "@." (the repo root) and runs the resolver with --min=@deps, minimizing every direct dependency and the Julia version to its floor. With the default lts channel the Julia floor is 1.10.0. The GROUP env var only selects which tests run; the resolver still resolves the entire root [targets].test set.

Mooncake 0.5.0 (the floor of Mooncake = "0.5", used by RecursiveArrayToolsMooncakeExt) declares julia = "1.10.8 - 1.10, 1.11.6 - 1". So when Mooncake is in the root test target and the resolver minimizes Julia to 1.10.0, Mooncake is uninstallable -> Unsatisfiable.

Fix: split AD into a dedicated off-downgrade group

Instead of pinning the downgrade Julia version (the prior approach on this branch), this removes the AD backends from the downgrade resolver at the source, following the same pattern as FunctionWrappersWrappers #59:

• The AD test files (adjoints.jl: Zygote/ForwardDiff/ReverseDiff; mooncake.jl: Mooncake) move from test/Core/ into a new dedicated AD group with its own sub-environment test/AD/Project.toml (with [sources] RecursiveArrayTools = {path = "../.."}, modeled on the existing NoPre group).
• test/test_groups.toml gains an [AD] group with in_all = false. The Downgrade.yml matrix runs only Core, so AD is never fed to the downgrade resolver.
• The AD backends (Mooncake/Zygote/ForwardDiff/ReverseDiff) are removed from the root [extras] and [targets].test. They remain [weakdeps] + [compat] for the extensions.
• Downgrade.yml is reverted to master (the julia-version: "1.11" pin is dropped).
• The AD tests still run, in their own CI group on a normal Julia (lts/1).
• The tupe -> type spellcheck fix is kept.

This dodges the Resolver<->Mooncake downgrade wall (StefanKarpinski/Resolver.jl#24 / the Mooncake >= 1.10.8 floor) by keeping Mooncake out of the resolution the downgrade lane performs, rather than working around it with a Julia-version pin.

Local verification

Faithful reproduction on a real Julia 1.10.0 binary (the exact patch the downgrade resolver minimizes to), resolving the merged root-test env with every non-stdlib dep pinned to its [compat] floor (=<floor>, mirroring julia-downgrade-compat, stdlibs skipped):

master  (Mooncake in [targets].test):
  Unsatisfiable requirements detected for package Mooncake [da2b9cff]:
   restricted to versions 0.5.0 by DowngradeProbe ... leaving only versions: 0.5.0
   restricted by julia compatibility requirements to versions: 0.3.0-0.4.137 or uninstalled -- no versions left
  => reproduces the master red

this PR (Mooncake removed from [targets].test):
  RESOLVED CLEANLY

And the AD group still loads and runs Mooncake on a normal Julia (1.10.11, the lts patch >= 1.10.8):

GROUP=AD  Pkg.test("RecursiveArrayTools")  ->  RecursiveArrayTools tests passed
  (RecursiveArrayToolsMooncakeExt precompiled, Mooncake v0.5.32; AD group 26/26 pass)

Please ignore until reviewed by @ChrisRackauckas.