Build failure using gcc 16.1.0

Bug in Gentoo: https://bugs.gentoo.org/975268

```
gcc --version
gcc (Gentoo 16.1.0 p1) 16.1.0
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
```

```
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h: In instantiation of 'void ktx::Reporter::fatal_usage(Args&& ...) [with Args = {const char (&)[26], const std::__cxx11::basic_string, std::allocator >&}]':
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:233:31: required from here
233 | report.fatal_usage("Unsupported format: \"{}\".", formatStr);
| ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:122:19: error: call to consteval function 'fmt::v11::fstring, std::allocator >&>((* & std::forward((* & args#0))))' is not a constant expression
122 | fmt::print(std::cerr, std::forward(args)...);
| ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/format.h:41,
from /tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/chrono.h:23,
from /tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/ostream.h:23,
from /tmp/KTX-Software-4.4.2.old/tools/ktx/utility.h:10,
from /tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:12:
/tmp/KTX-Software-4.4.2.old/tools/ktx/command.h:122:19: in 'constexpr' expansion of 'fmt::v11::fstring, std::allocator >&>((* & std::forward((* & args#0))))'
/tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/base.h:2736:53: in 'constexpr' expansion of 'fmt::v11::basic_string_view(((const char*)s))'
2736 | if (FMT_USE_CONSTEVAL) parse_format_string(s, checker(s, arg_pack()));
| ~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
/tmp/KTX-Software-4.4.2.old/external/fmt/include/fmt/base.h:548:5: error: 'args#0' is not a constant expression
548 | while (*s++) ++len;
| ^~~~~
```

The following code no longer builds with GCC 16.1:
```cpp
template
inline void do_print(Arg&&...a )
{
fmt::print(std::cerr, std::forward(a)...);
}
```

It can be fixed with
```cpp
template
inline void do_print(fmt::format_string f, Arg&&...a )
{
fmt::print(std::cerr, f, std::forward(a)...);
}
```

This is however not sufficient and we get other fmt related failures in `formatOptionalFileOffset`, `DiffHexFixedWidth`.

I am not sure if this is a bug in KTX-Software, libfmt or gcc.
Replacing the bundled libfmt with system one 12.1.0 does not fix the issue.

Reproduce the GCC 16.1.0 failure from the report and inspect tools/ktx/command.h, especially do_print and the calls around formatOptionalFileOffset and DiffHexFixedWidth. Compare the bundled external/fmt usage with the reported format_string approach, then verify that the KTX tools build successfully with GCC 16.1.0 and that the reported fmt-related failures are resolved.

Written by the indexing model from the issue text.