Use SciMLTesting v1.2 (folder-based run_tests)

[ChrisRackauckas-Claude]

Converts ConcreteStructs.jl's test suite to the SciMLTesting v1.2 folder-discovery model.

What changed

• runtests.jl becomes using SciMLTesting; run_tests() (bare folder-discovery).
• Core split from the monolithic core.jl into self-contained top-level files — unit_tests.jl, end_to_end_tests.jl, issue_3.jl, commutation_tests.jl — each run isolated in its own @safetestset by folder discovery.
• The previously unconditional "Invalid usage error path" testset becomes a self-contained Core file invalid_usage_tests.jl. It is a @concrete macro-correctness unit test and belongs to Core; CI continues to exercise it on the Core lane.
• QA stays at test/qa/ with its existing qa.jl + Project.toml sub-env.
• SciMLTesting + SafeTestsets added to the main test env and the QA sub-env; Pkg dropped from the test env (no test file uses it; the harness owns all Pkg ops). test_groups.toml unchanged.
• Removed the checked-in test/Manifest.toml (it is gitignored and was stale after the dep change; Pkg re-resolves it fresh).

Verification

• TOML.parsefile on every Project.toml + test_groups.toml: OK.
• Meta.parseall on runtests.jl and every moved/new test file: OK.
• Core's assertion set is preserved exactly: 42 @test/@test_throws before and after (including the moved invalid-usage check).
• Drove the real SciMLTesting v1.2.0 run_tests folder-discovery dispatcher over every GROUP value: Core runs the 5 top-level files isolated; QA activates the qa/ sub-env and runs qa.jl; All runs Core (QA is its own CI lane per the v1.2 folder semantics). CI dispatches Core and QA as separate matrix lanes from test_groups.toml.
• All authored/changed Julia files are Runic-clean.

This supersedes the v1.0.0 harness form previously on this branch and the canonicalize-safetestset PR.

Ignore until reviewed by @ChrisRackauckas.