fix: let the accessors be called on a temporary

[oberbichler]

Problem

The accessors take their object by deducing this, declared as an lvalue reference — which cannot bind to a temporary. So a result could not be read from the expression that produced it:

(a * b).f()        // error: candidate function not viable:
sqrt(a).g(0)       //        expects an lvalue for object argument
a.sin().h(0, 0)

Every C++ caller had to introduce a named variable for every intermediate result. Six accessors were affected: data, ptr, f, g, h(i) and h(i, j).

This came in with the deducing-this refactor of the C++23 modernisation, which replaced the duplicated const/non-const overload pairs. The old pairs had a const overload that bound to temporaries; the single deducing-this version does not.

Fix

The object parameter becomes a forwarding reference — this auto &&self instead of this auto &self — which binds to lvalues, const lvalues and temporaries alike. One character per accessor.

SScalar is unaffected: its f() and d() return by value.

Caveat, and it is documented in the header

As with std::vector::operator[], a reference obtained from a temporary is only valid within the full expression:

const auto &r = (a * a).f();   // dangles, and no compiler diagnoses it

I checked: -Wall -Wextra -Wpedantic emits nothing for that. The note sits where the accessors are declared.

What did not change

The writable path: result.f() = value on an lvalue still works — the factories rely on it — and so does reading through a const lvalue. Verified separately as well as by the suite.

Tests

The property is stated twice over.

Callability, as concepts over all four combinations of order and sizing — 18 assertions, all of which failed before the change:

test_variants.cpp:135: ERROR: CHECK( ReadsValue<T> ) is NOT correct!
test_variants.cpp:136: ERROR: CHECK( ReadsGradient<T> ) is NOT correct!
test_variants.cpp:137: ERROR: CHECK( ReadsData<T> ) is NOT correct!
test_variants.cpp:138: ERROR: CHECK( ReadsPointer<T> ) is NOT correct!
test_variants.cpp:141: ERROR: CHECK( ReadsHessian<T> ) is NOT correct!

Deliberately expressed as concepts rather than as direct calls, so the red state is a failing assertion rather than a build error.

Values, read from temporaries and compared against the expectations the surrounding cases already use — including a chained read, a.sin().exp().f(), which is the case that makes this worth fixing at all.

Verification

• C++ 106 test cases, 1362 assertions — Debug and Release with -Werror, and under clang with ASan+UBSan
• Python 239 passed
• clang-tidy unchanged at 50; clang-format --Werror and ruff format --check clean

Notes

Independent of #84 and #85 — this touches DDScalar's accessors, those two touch SScalar. Based on main, merges in any order.

Found while measuring for the SScalar work: a benchmark of mine would not compile, and the compiler error turned out to be a defect in the library rather than in the benchmark.