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

Read the options of a project from catch3.json

macOS runners are the bottleneck of Progmasoft's CI: an organization
runs at most five macOS jobs at a time, shared by every repository, and a
pull request of Catch3 asked for about twenty of them. Every workflow
also listened to both push and pull_request, so each pull request ran
everything twice, once for its branch and once for the pull request.

macOS now has one job, as in the Tala compiler: macOS 26 on Apple
Silicon, Debug, C++17, with AddressSanitizer and UndefinedBehaviorSanitizer
over the most-tests preset, examples and extra tests. The C++14 and C++17,
Debug and Release matrix stays on Ubuntu and Fedora, where it already ran.
ThreadSanitizer, which ran only on macOS, moves to a new Ubuntu job.

The build systems that tested Windows and macOS (Bazel, Meson, Xmake,
Premake) test Windows and Ubuntu instead, with Clang, so their Unix side
is still covered and Linux is covered for the first time. The jobs that do
not depend on the platform leave macOS: CodeQL (build mode none), the
header and license checks, and native coverage, which now builds with
the Clang and LLVM tools of Ubuntu instead of those of Xcode.

Pushes build only devel; pull requests build their branches. The weekly
CodeQL schedule stays.

The required check "Bazel on macos-15" becomes "Bazel on ubuntu-26.04" in
the branch protection and the ruleset when this is merged.

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