Enable -Wall -Wextra -Werror: first-party baseline + gate generated code (compile_check_driver), with a cross-repo golden-compile follow-up #89

Motivation

The project currently enforces no warning flags — no -Wall/-Wextra/-Werror in CMakeLists.txt, tests/CMakeLists.txt, or the CI workflow (.github/workflows/build.yml runs a bare cmake -B build -DCMAKE_BUILD_TYPE=...). Two PR #88 review rounds surfaced warnings in generated code (-Wunused-variable on a state-independent rate's <state> local; clang -Wunused-private-field on an unreferenced m_<param>) that would break a downstream -Werror build. Those were fixed with [[maybe_unused]], but nothing guards against a regression — because codegen never compiles the affected output under a warning gate.

Proposal (three parts, increasing cost)

1. First-party warning baseline (cheap, do first)

Add -Wall -Wextra -Wpedantic to the first-party targets (numsim_codegen + numsim_codegen_tests), and -Werror in CI. Scope it to first-party only — fetched deps (numsim-cas, tmech, Eigen, gtest) must stay SYSTEM/-isystem so their header warnings don't trip -Werror (CAS is already SYSTEM YES; verify tmech/Eigen are -isystem). This catches warnings in codegen's own source. Note: it does not catch generated-code warnings.

2. Gate the GENERATED standalone code (the high-value part)

compile_check_driver already compiles codegen-generated standalone headers (CompileCheck, PiecewiseCheck, …) in-repo. Add -Wall -Wextra -Werror to that target so generated standalone code is warning-clean as a CI invariant. This directly regression-guards the PR #88 [[maybe_unused]] fixes for the standalone path — the single most valuable addition, and it needs no extra toolchain.

3. Gate the generated numsim-materials material (cross-repo follow-up)

The NumSimMaterialTarget output can't be compiled in this repo (it needs the numsim-materials headers + g++-14, since numsim-materials uses <print>). Warning-gating it requires the cross-repo golden-compile CI job (compile a generated material against a pinned numsim-materials SHA with -Wall -Wextra -Werror) — see the separate CI-gate follow-up. Until that exists, the round-3 [[maybe_unused]] fixes are only guarded by manual cross-repo spikes.

Caveats

• Scope -Werror to first-party. Deps' headers warn under -Wextra; keep them SYSTEM.
• Validate across the matrix (gcc-14 + clang-18) — they diverge (e.g. -Wunused-private-field is clang-only). A warning clean on one may fail the other.
• Prefer -Werror in CI, plain -Wall -Wextra locally (e.g. option(NUMSIM_CODEGEN_WERROR ...) defaulting OFF, ON in CI), so a newer local compiler's novel warnings don't block day-to-day builds.

Acceptance

• First-party targets build clean under -Wall -Wextra on both matrix compilers; -Werror enforced in CI.
• compile_check_driver compiles all generated standalone headers under -Wall -Wextra -Werror.
• Cross-repo golden-compile gate tracked separately for the numsim-materials target.

Origin: PR #88 review rounds 2–3 (generated-code -Werror safety).