Move the doctests into their own test group

CHANGELOG.md

@@ -24,6 +24,7 @@ breaking release).
 
 ### Changed
 
+- `Pkg.test()` no longer runs the doctests. `test/quality/doctests.jl` is now the group `doctests`, which an empty `ARGS` does not run; `Pkg.test(test_args = ["doctests"])` runs it. In CI the Doctests job stays their runner, so the test matrix no longer runs them a second time.
 - **CI uploads coverage from the `Julia 1 - ubuntu-latest` job instead of `Julia min`, and a test job saves the Julia cache only when it succeeds.**
 - **The package needs Julia 1.12 or later** (`[compat] julia = "1.12"`, was `"1.11"`). On Julia 1.11.9, the gradient of an `LNNLoss` with respect to the network parameters stops with `StackOverflowError` in type inference ("Internal error: during type inference of … Encountered stack overflow") on macOS and Windows; the `Julia min` job on Linux passes. The overflow comes from SymbolicUtils 4.49.0, whose `rrule` for `SymbolicUtils.Code.create_array` returns an exactly typed tuple of tangents, one per array element; Zygote's reverse pass through the generated Hessian of `lagrangian_acceleration` is then inferred far deeper. With the 4.48.0 rule restored, or in a task with a 64 MB stack, the same gradient passes on 1.11.9; Julia 1.12.7 and 1.13.1 pass it on the root task. This closes #327: the `Julia min` jobs on macOS and Windows had failed in the *Lagrangian Neural Network* testset since 2026-10-01, and every testset after it was never run there.
 - **`test/Project.toml` has no `[compat]` entry for a dependency of the package.** The entries for AbstractNeuralNetworks, GeometricOptimizers, HDF5, KernelAbstractions, LinearAlgebra, NeuralNetworkParameters, Random and Zygote are gone: the test environment contains the package, so the bounds of `Project.toml` apply there, and an entry in `test/Project.toml` can only repeat or narrow them. `NeuralNetworkParameters = "0.3"` was narrower than the package's `"0.3, 0.4"`.

test/runtests.jl

@@ -88,6 +88,8 @@ end
 if "slow" in GROUPS
     @safetestset "Lagrangian Neural Network" include("architectures/lagrangian_neural_network_tests.jl")
     @safetestset "Test symplecticity of the sympnet upscaling layer" include("layers/sympnet_upscaling.jl")
+end
+if "doctests" in GROUPS
     @safetestset "Doctests" include("quality/doctests.jl")
 end
 if "metal" in GROUPS