Move JET into a separate QA test group

[ChrisRackauckas-Claude]

Summary

PR #133's CI fails on Julia pre (1.13.0-beta3) at Pkg.resolve:

JET [c3a54625] log:
├─possible versions are: 0.0.1 - 0.11.3 or uninstalled
├─restricted to versions 0.9 - 0.11 by project
└─restricted by julia compatibility requirements to versions: uninstalled — no versions left

JET 0.11.3 (its current max) has no version compatible with Julia 1.13, so the whole main test environment cannot be instantiated. JET is a QA tool, not a runtime dependency — blocking the main matrix on JET's release cadence is the wrong tradeoff.

This PR moves JET into a dedicated QA group, mirroring how SciML/Sundials.jl ships JET / Aqua / ExplicitImports.

Changes

• Add test/qa/Project.toml (deps: JET, MultiScaleArrays, Test) and test/qa/runtests.jl (runs JET.report_package, asserts zero reports).
• Delete test/jet_tests.jl and remove the corresponding @testset from test/runtests.jl.
• Drop JET from main Project.toml [compat], [extras], and the test target.
• Add .github/workflows/QA.yml that runs test/qa/runtests.jl on Julia 1 and lts (skips pre).

Local verification

Julia 1.12.6:

QA group:

$ julia --project=test/qa test/qa/runtests.jl
Test Summary:     | Pass  Total   Time
Quality Assurance |    1      1  11.5s

Main test suite (no JET):

Tuple Nodes | … | Pass
Bisect Search Tests | … | Pass
Indexing and Creation Tests |  133  133  2m42.1s
Values Indexing |   55   55  0.1s
Get Indices Tests |   19   19  0.2s
Additional Fields Test (compile-only) | 46.4s
Dynamic DiffEq Tests |    3    3  10m53.2s   (SDE: SRIW1, SRA1, RKMil, EM)
Single Layer DiffEq Tests |    4    4  3m09.4s
New Nodes Tests |    1    1  7.3s
     Testing MultiScaleArrays tests passed

Test plan

• Tests.yml matrix (1, lts, pre) all pass — pre should now succeed because main test deps no longer include JET.
• QA.yml matrix (1, lts) passes.
• test (alldeps, 1.10) (Downgrade) still passes at the new minimum.

Companion to #134 (compat lower-bound bump). Either can land first.