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.

This was originally motivated by `REQUIRE((a <=> b) == 0)` no
longer compiling using MSVC. After some investigation, I found
that they changed their implementation of the zero literal
detector from the previous pointer-constructor with deleted
other constructors, into one that uses `consteval` constructor
from int.

This breaks the previous detection logic, because now
`is_foo_comparable<std::strong_ordering, int>` is true, but
actually trying to compare them is a compile-time error...
The solution was to make the decomposition `constexpr` and rely
on a late C++20 DR that makes it so that `consteval` propagates
up through the callstack of `constexpr` functions, until it either
runs out of `constexpr` functions, or succeeds.

However, the default handling of types in decomposition is to
take a reference to them. This reference never becomes dangling,
but because the constexpr evaluation engine cannot prove this,
decomposition paths taking references to objects cannot be
actually evaluated at compilation time. Thankfully we already
did have a value-oriented decomposition path for arithmetic types
(as these are common linkage-less types), so we could just
explicitly spell out the `std::foo_ordering` types as also being
supposed to be decomposed by-value.

Two more fun facts about these changes
 1) The original motivation of the MSVC change was to avoid
    trigering a `Wzero-as-null-pointer-constant` warning. I still
    do not believe this was a good decision.
 2) Current latest version of MSVC does not actually implement the
    aforementioned C++20 DR, so even with this commit, MSVC cannot
    compile `REQUIRE((a <=> b) == 0)`.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1

The compile time improvements from using combined TUs mostly isn't
worth the annoyance they cause with various IDE shortcuts, like
when switching between header and its impl. file.

Splitting them apart also fixes the issue of empty subdirs being
installed due to `foo/internal` folders that only contained the
combined TUs and no headers.

Closes #2457
Closes #2463

This should improve the compilation times by decreasing the number
of TUs compiled, without making overly big TUs that would cause
problems with heavy-tailed compilation times.

There is one "combined TU" for the top level part, and each subpart,
except for Reporters, which currently do not have any trivial TUs.

This is both a really big and a really small commit. It is small in
that it only contains renaming, moving and modification of include
directives caused by this.

It is really big in the obvious way of touching something like 200
files.

The new rules for naming files is simple: headers use the `.hpp`
extension. The rules for physical file layout is still kinda in
progress, but the basics are also simple:
 * Significant parts of functionality get their own subfolder
   * Benchmarking is in `catch2/benchmark`
   * Matchers are in `catch2/matchers`
   * Generators are in `catch2/generators`
   * Reporters are in `catch2/reporters`
   * Baseline testing facilities are in `catch2/`
 * Various top level folders also contain `internal` subfolder,
   with files that users probably do not want to include directly,
   at least not until they have to write something like their own
   reporter.
    * The exact files in these subfolders is likely to change later
      on

Note that while some includes were cleaned up in this commit, it
is only the low hanging fruit and further cleanup using automatic
tooling will happen later.

Also note that various include guards, copyright notices and file
headers will also be standardized later, rather than in this commit.