Dividing a long long by 10.0 is an implicit conversion, which fails in a
project that builds its tests with -Wimplicit-int-float-conversion and
warnings as errors, as Tala does.

Move to the Progmasoft Linking Exception, Version 2.0

Build Catch3 as C++17

Version 1.1 left programs built with Progmasoft tools under MPL-2.0 duties
for the code the tools place in them, and left other cases unclear.
Version 2.0 adds a generated code permission, covers runtime and standard
libraries, compiler transformations, interfaces and examples, and settles
how a file is identified and how to tell recipients about source.

Every project SPDX expression moves to AdditionRef-Progmasoft-Exception-2.0.
The older texts stay, because released versions name them.

A lambda in the POSIX half of Process.cpp captured a structured binding,
which needs C++20 and which a Windows build never compiles. The files in
extras still held the C++20 sources, so they are generated again, and the
generator no longer says the runner needs C++20.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

C++17 is still the widest common target, and a library built on it stays
usable from C++17 and every later standard, while C++20 would have left
out many users.

Concepts become enable_if on named traits in Detail/TypeTraits.hpp, which
also replaces std::remove_cvref_t and std::identity. std::span becomes
std::vector references, std::bit_cast becomes static_cast and memcpy,
template lambdas become helper templates, and std::erase_if, contains,
starts_with and ends_with have their C++17 forms. Random::Between no
longer uses a lambda over a constexpr local, which MSVC rejects outside
/permissive-, so the headers work under its default mode too.

Tests give up designated initializers, which MSVC refuses in C++17. CI
builds the matrix in C++17 and C++20, since the headers promise both.

Give macOS one CI job and stop building pull requests twice