default_init results in a memset with GCC16

Start with the small_vector constructor using boost::container::default_init and reproduce the assembly shown with GCC16.0.1, GCC15.2, and Clang 22.1 under -std=c++26 -O3. Compare the generated code and trace whether the memset comes from the container implementation or the compiler; done means identifying the cause and documenting a fix, workaround, or confirmed compiler issue.

It's perhaps a little early to open an issue for this but since I'm already toying with GCC16, I was surprised to see that it now generates a memset to zero init even when default_init is used.

    boost::container::small_vector<float, 100> bar(100, boost::container::default_init);

    for(auto b : bar)
        std::cout << b;

GCC16.0.1 (-std=c++26 -O3):

        push    r12
        mov     edx, 424
        xor     esi, esi
        push    rbp
        push    rbx
        sub     rsp, 432
        mov     rdi, rsp
        lea     r12, [rsp+24]
        lea     rbp, [rsp+424]
        call    "memset" ; zero init 400 bytes
        mov     eax, 100
        mov     QWORD PTR [rsp], r12
        mov     rbx, r12
        movq    xmm0, rax
        punpcklqdq      xmm0, xmm0
        movups  XMMWORD PTR [rsp+8], xmm0

32-bit (only the relevant bit):

        lea     eax, [ebp-440]
        push    412 ; count
        push    0 ; ch
        push    eax ; dest
        call    "memset"

GCC15.2 (-std=c++26 -O3):

        push    r12
        mov     eax, 100
        push    rbp
        movq    xmm0, rax
        push    rbx
        punpcklqdq      xmm0, xmm0
        sub     rsp, 432
        lea     r12, [rsp+24]
        movups  XMMWORD PTR [rsp+8], xmm0
        lea     rbp, [rsp+424]
        mov     QWORD PTR [rsp], r12
        mov     rbx, r12

Clang 22.1 (-std=c++26 -O3):

        push    r15
        push    r14
        push    rbx
        sub     rsp, 432
        lea     r14, [rsp + 32]
        mov     qword ptr [rsp + 24], 100
        mov     qword ptr [rsp + 8], r14
        mov     qword ptr [rsp + 16], 100
        mov     r15d, 24
        mov     rbx, qword ptr [rip + std::cout@GOTPCREL]

I'm not yet sure if this is a compiler regression, an issue with the containers, a skill issue or something else. Again, happy to close it if it's too soon. :)