Move heavy AD backends out of root [extras] into group envs

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas. This is an automated draft.

What & why

FastPower's optional AD backends (Enzyme, ForwardDiff, Mooncake, ReverseDiff, Tracker) plus their test-only support deps (EnzymeTestUtils, StableRNGs) lived in the root Project.toml's [extras]/[targets].test. Under the folder-model harness both the Core group (top-level test/*.jl) and the Enzyme group folder run in the main test env, so this pulled the full joint AD graph into every root/Core resolve — including the Downgrade lane, which had to floor-resolve all five heavy backends together against the base package. That is exactly the joint-graph floor wall the group-env migration removes.

Each backend is moved into the group env whose files actually using it:

• test/AD/Project.toml (new) — ForwardDiff/ReverseDiff/Tracker/Mooncake for test/AD/other_ad_engines_tests.jl. That file was a loose top-level Core file (using ForwardDiff, ReverseDiff, Tracker, Mooncake); it is relocated into its own AD group folder (declared in test_groups.toml) so those backends leave the root/Core env.
• test/Enzyme/Project.toml (new) — Enzyme/EnzymeTestUtils/StableRNGs for the existing test/Enzyme/ group folder, which previously had no Project.toml and so resolved against the root env.

Root [extras]/[targets].test now keeps only SafeTestsets/SciMLTesting/Test (harness + Core deps). Weakdep [compat] entries (Enzyme, ForwardDiff, Mooncake, ReverseDiff, Tracker) stay because they still gate package extensions; the extra-only compats (EnzymeTestUtils, StableRNGs) are dropped so they don't dangle.

No runtests.jl change is needed — the folder-discovery harness (run_tests()) auto-activates each group folder's Project.toml (activate_group_env). The AD group is added to test/test_groups.toml, so the centralized grouped-tests.yml CI matrix picks it up automatically.

Downgrade effect (jdc-52 harness, --min=@deps, fresh depot each)

BEFORE (main): base resolves, then Independently downgrading 5 weakdep extension(s): Enzyme, ForwardDiff, Mooncake, ReverseDiff, Tracker — all five floor-checked in the root joint graph (writes Manifest-{Enzyme,ForwardDiff,Mooncake,ReverseDiff,Tracker}.toml). EnzymeTestUtils + StableRNGs also present in the base resolve.

AFTER (this branch): merged root project has only Test/SafeTestsets/SciMLTesting extras; base Successfully resolved minimal versions; no Independently downgrading … step at all — the root/Core Downgrade lane no longer floor-resolves any AD backend. No per-backend manifests written; base manifest contains none of the seven moved packages.

Verification (all run locally)

• test/AD env: resolves + loads ForwardDiff 1.4.1 / ReverseDiff 1.17.0 / Tracker 0.2.38 / Mooncake 0.5.36; all FastPower*Ext precompile.
• test/Enzyme env: resolves + loads Enzyme 0.13.173 / EnzymeTestUtils 0.2.8 / StableRNGs 1.0.4; FastPowerEnzymeExt precompiles.
• GROUP=Core Pkg.test: passes (1200 + 5 assertions), root env carries no AD backend.
• GROUP=AD Pkg.test: AD/other_ad_engines_tests.jl 4/4 via auto-activated env.
• GROUP=Enzyme Pkg.test: enzyme_forward_tests.jl 52/52 + enzyme_reverse_tests.jl 36/36 via auto-activated env.

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