MooseMaterialTarget output is never compiled — add a MOOSE-stub compile gate #132

From the 2026-08-02 full-repo review (finding F-04).

Evidence: all six compile-and-run pipelines in tests/CMakeLists.txt emit via StandaloneCxxTarget or NumSimMaterialTarget (the "MOOSE Hencky" gate at line ~169 actually emits via StandaloneCxx); .github/workflows/build.yml never touches MOOSE. Every MOOSE test (MooseTargetTest.cpp, IntegrationTest.cpp:117-187, AlgorithmicTangentTest.cpp:126-238) is substring-only.

Impact: MOOSE is the one shipping backend with zero compile-level backstop, and the one referencing a large external API surface (Material, InputParameters, MaterialProperty<T>, RankTwoTensor/RankFourTensor, registerMooseObject). A boilerplate typo in the emitted .h/.C ships green.

Proposal: check in a minimal stub of exactly the MOOSE surface the emitted code uses; at build time, generate representative materials (linear elastic, Hencky/spectral, tangent-carrying, local-Newton) and compile the emitted .C against the stub with -Wall -Werror. Syntax/semantics gate only — no MOOSE behavior emulation needed. A later step can add a tiny harness to drive computeQpProperties() and FD-check _Jacobian_mult (that part = #12).

Related: #12 (runtime parity against mock RankTwoTensor — this issue is the cheaper compile-level prerequisite).