fix: make the array overload of variables() deducible

[oberbichler]

Problem

template <index T>                                  // index = std::ptrdiff_t
static std::conditional_t<...> variables(const std::array<Scalar, T> &values)

std::array is sized by std::size_t, so deducing a ptrdiff_t non-type parameter from the argument always fails:

error: no matching function for call to 'variables'
note: candidate function not viable: no known conversion from 'std::array<double, 2>'
      to 'const std::vector<Scalar>' for 1st argument
note: candidate template ignored: substitution failure: deduced non-type template
      argument does not have the same type as the corresponding template parameter
      ('unsigned long' vs 'index' (aka 'long'))

The C++ usage example in the README did not compile. The overload was only ever reachable from the bindings, which passed the size explicitly and therefore never exercised deduction:

T::template variables<T::static_size()>(values)

Fix

Declare T as std::size_t. Three small consequences:

• The explicit workaround in common.h goes away — T::variables(values) now deduces.
• static_assert(index(T) == TSize) compares as index, so both sides keep the same signedness (relevant now that -Wall -Wextra is on since ci: add a sanitizer job and turn on compiler warnings #75).
• The local array is sized by T instead of TSize, which the assert has just established to be equal, and avoids a signed→unsigned conversion in a template argument.

Verification

The README snippet now compiles verbatim. Rather than retyping it, I extracted the ```cpp block straight out of README.md and built it with -Wall -Wextra -Wpedantic -Werror:

f   = -6
df/dx = -4
df/dy = 1
d²f/dx² = -2.66667

which is exactly what its inline comments claim.

• C++ 45/45 test cases, 449 assertions — Debug, Release, and the clang ASan+UBSan job, all with -Werror
• Python 166 passed; D3Scalar.variables([1,2,3]) and DDScalar.variables([1,2]) verified to still produce the right gradients through the simplified binding
• clang-format --Werror and ruff format --check clean

Tests

TEST_CASE("README example") pins the example's values, so it cannot silently stop compiling or start lying again. Its Hessian is the same computation the Python quickstart shows in the README ([[-2.667, 1.333], [1.333, -0.667]]), so those numbers are covered by the same test.

TEST_CASE("Variables from an array") exercises both branches of the conditional return type — std::array<Type, T> for static and std::vector<Type> for dynamic. Neither had any C++ coverage before, which is how the deduction failure survived: the only caller passed the size explicitly.

Notes

A durable version of the README check would extract and compile the snippets in CI rather than duplicating them in a test. Worth doing once there are more of them; the pinned test case covers this one.

Based on main with #75 merged. Independent of #76 and #77 (both still open).