Uniformize monorepo structure to OrdinaryDiffEq canonical style

[ChrisRackauckas-Claude]

Uniformizes DataDrivenDiffEq.jl to the OrdinaryDiffEq.jl canonical monorepo style: the lib/* sublibraries become a real path-graph project model tested through the centralized SublibraryCI/DowngradeSublibraries workflows, and the root track is reduced to the root package's own groups.

Canonicalization done

[sources] path graph (the MAJOR fix)

• Each sublibrary (DataDrivenDMD, DataDrivenSR, DataDrivenSparse, DataDrivenLux) gains [sources] DataDrivenDiffEq = {path = "../.."}. The sublibraries are leaves that depend on the in-repo root DataDrivenDiffEq; without [sources] they resolved against the registered release instead of the PR checkout. The root does not re-export the sublibraries (no sublib deps in root [deps]), so per the true dependency direction no cyclic root→leaf [sources] edges were added.

Single group env var (DATADRIVENDIFFEQ_TEST_GROUP) threaded consistently

• SublibraryCI.yml: with: { group-env-name: DATADRIVENDIFFEQ_TEST_GROUP, check-bounds: auto }, secrets: inherit.
• DowngradeSublibraries.yml: group-env-name: DATADRIVENDIFFEQ_TEST_GROUP, group-env-value: Core; no allow-reresolve (v1 removed that input); skip list = stdlibs + the path-developed sibling DataDrivenDiffEq; sublibraries auto-discovered.
• Every sublibrary test/runtests.jl now reads get(ENV, "DATADRIVENDIFFEQ_TEST_GROUP", get(ENV, "GROUP", "All")) and runs its functional suite on Core/All. DataDrivenSparse and DataDrivenSR previously had no group gate at all.

Root track

• CI.yml: dropped the hardcoded sublibrary GROUP rows (DataDrivenDMD/SR/Sparse/Lux) so sublibraries are tested only through SublibraryCI's project model and are not double-tested; kept a root-only group set (Core, nopre); coverage scoped to src.
• Downgrade.yml: group: Core, no allow-reresolve; added a concurrency block.
• test/runtests.jl: replaced the ad-hoc dispatcher with an OrdinaryDiffEq-style _detect_sublibrary_group dispatcher — if GROUP names lib/<X> (optionally lib/<X>_<TESTGROUP>) it activates the sublibrary, develops its [sources] transitively on Julia < 1.11, sets DATADRIVENDIFFEQ_TEST_GROUP, and Pkg.tests it; otherwise it runs the root @safetestset groups (All/Core/Downstream) or the nopre JET env.

test_groups.toml

• Added [Core] on ["lts","1.11","1","pre"] to each sublibrary so the default QA group (no QA/Aqua/JET suite exists in these sublibraries) is not spawned.

Invariants preserved

• No [compat] floor lowered; all julia = "1.10" floors kept.
• No test skipped/@test_broken/@test_skip/commented-out; no tolerance loosened.
• No bare test/Project.toml exists (the per-group test/nopre/Project.toml is the canonical isolated-env pattern, kept as-is — same as OrdinaryDiffEq's test/qa/Project.toml etc.).
• No ReTestItems/@testitem present anywhere (the repo already used SafeTestsets).

Verified locally (isolated depot, Julia 1.10 + 1.11)

• TOML-parsed every changed Project.toml; confirmed every [targets].test dep appears in [extras]/[deps]; confirmed each sublib has [sources] DataDrivenDiffEq = {path="../.."}; parsed all four test_groups.toml.
• YAML-parsed CI.yml, SublibraryCI.yml, DowngradeSublibraries.yml, Downgrade.yml.
• Instantiated the root and a representative sublibrary; [sources] resolves the in-repo root natively on 1.11.
• Ran the root Core group (Julia 1.10): All | 260 260 — Testing DataDrivenDiffEq tests passed.
• Ran DataDrivenSparse Core via the faithful SublibraryCI path (Pkg.test + DATADRIVENDIFFEQ_TEST_GROUP=Core, --check-bounds=auto, Julia 1.11, [sources] native): Basic Sparse Regression 33/33, Pendulum 48/48, Michaelis Menten 9/9, Cartpole 4/4 — Testing DataDrivenSparse tests passed.
• Confirmed the QA group no-ops cleanly for a sublibrary (Pkg.test passes with zero test sets).
• Ran Runic on all changed .jl; all clean.

What CI must confirm (not run locally)

• The other three sublibraries (DataDrivenDMD, DataDrivenLux, DataDrivenSR) under SublibraryCI across lts/1.11/1/pre (only DataDrivenSparse was run locally as the representative).
• The centralized sublibrary-project-tests.yml@v1 matrix discovery / coverage upload.
• DowngradeSublibraries and Downgrade downgrade resolution.
• The root nopre (JET) group.

Ignore until reviewed by @ChrisRackauckas.