Replace every (void) cast with [[maybe_unused]]

[petlenz]

Project style: (void) casts are out, [[maybe_unused]] is in. Sweeps all 61 sites on main.

The sites split into two kinds and get different treatments:

Unused variable / parameter (14 sites) — [[maybe_unused]] moves to the declaration and the (void)x; line is deleted:

for ([[maybe_unused]] auto const &t : model.tangents()) { ... }
[[maybe_unused]] cas::expression_holder<cas::scalar_expression> const *x = nullptr;
[[maybe_unused]] inline auto diff_tensor_wrt_scalar(
    [[maybe_unused]] ... const &expr, [[maybe_unused]] ... const &arg)

Discarding a [[nodiscard]] return (47 sites) — an attribute cannot annotate an expression, so these become a named declaration:

EXPECT_THROW([[maybe_unused]] auto const discarded = m.emit_compute_function(),
             std::runtime_error);
[[maybe_unused]] auto const K = m.add_parameter("K", 1.0);

Discard variables are named after the symbol being declared where one exists (K, alpha, eps_p), discarded otherwise.

Two incidental cleanups found by the sweep:

• src/targets/numsim_material.cpp — (void)i; was dead; i is used two lines above as outputs[i]. Deleted rather than annotated.
• tests/IntegrationTest.cpp — the natural name a for one discard collided with a lambda's for (auto const &a : args) under -Wshadow; renamed a_sv.

Verification: git grep '(void)' over *.cpp/*.h returns nothing. Debug build clean under -Werror (which has -Wshadow and -Wunused on), 298/298 unit tests, 337/338 ctest. The one ctest failure, NumSimMaterialEndToEnd.J2PathDependentRetainsPlasticStrainOnUnload, reproduces identically on unmodified main — pre-existing, not from this change.

Open PRs that add their own (void) sites (#120, #121, #122, #123, #127) get the same treatment on their branches separately.