Review Catch2 devel through 08092139210881f01a34e60c5f2fd6afa0ea4024,
the commit of 3.16.1, and apply the eleven changes since the last
reviewed revision in upstream order:

  d7683484  better message when a reporter cannot be initialized
  ea5020f0  custom argument validation in every first-party reporter
  aa01559e  throwing-reporter test also throws in assertionEnded
  45cd3987  noexcept on handleUnexpectedInflightException
  6adbd539  MSVC optimizations enabled again after the benchmark headers
  e87f2882  no quadratic scaling for NTTP test registration
  73d546cd  REQUIRE expands its arguments fewer times under GCC and Clang
  b710039a  decomposer operators as plain functions
  f899467b  type-erased streaming of reconstructed expressions
  8f0e70ca  retired macos-14 runner dropped
  08092139  version 3.16.1 and its release notes

All of them applied to the sources without a conflict. The amalgamated
distribution under extras/ conflicted because it is generated; it is
generated again from the sources of this repository, which also brings
it up to date with fork changes it had not picked up: the list-based
stream pool, the Clang diagnostic boundary and the cassert include.

No file under src/Progmasoft changes. The fork-owned texts that name
the engine version say 3.16.1, and docs/upstream-status.md records the
review. Upstream files keep their names, licenses and compiler options.

Built with Clang 22 on Windows and tested: 148 of 149 tests pass. The
one that fails, Reporters::CrashInJunitReporter, fails the same way on
devel without this change with that compiler.

Not being able to filter generators to specific element has been regularly
causing problems. It was possible to use a dynamic section to run tests
for specific element in a generator, at least if the element had a nice
string representation, but the test case would still run once per element
in the generator.

With this change, it is possible to have the generator return only one
specific element, and do so based on the index, rather than the string
representation of the element. This enables simple debugging of tests
that fail for specific generator element.

This will be useful later to implement warning on infinitely
running `GENERATE` expressions.

* Add new SKIP macro for skipping tests at runtime

This adds a new `SKIP` macro for dynamically skipping tests at runtime.
The "skipped" status of a test case is treated as a first-class citizen,
like "succeeded" or "failed", and is reported with a new color on the
console.

* Don't show "skipped assertions" in console/compact reporters

Also extend skip tests to cover a few more use cases.

* Return exit code 4 if all test cases are skipped

* Use LightGrey for the skip colour

This isn't great, but is better than the deep blue that was borderline
invisible on dark backgrounds. The fix is to redo the colouring
a bit, including introducing light-blue that is actually visible.

* Add support for explicit skips in all reporters

* --allow-running-no-tests also allows all tests to be skipped

* Add docs for SKIP macro, deprecate IEventListener::skipTest

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>