License Catch3's Progmasoft files under LGPL-3.0-or-later

Every Progmasoft-authored file moves from MPL-2.0 WITH
AdditionRef-Progmasoft-Exception-2.0 to LGPL-3.0-or-later, from 0.5.0
on. Catch2-originated files keep BSL-1.0, and the vendored yyjson and
pugixml keep MIT.

- LICENSES/ holds the LGPL-3.0 text and the GPL-3.0 text it builds on,
  both as published by the FSF; the MPL-2.0 and Progmasoft Exception
  texts go, since no file uses them any more.
- Five files that came from Catch2 and were rewritten by Progmasoft
  (.clang-format, the issue and pull request templates, SECURITY.md)
  keep Catch2's notice next to Progmasoft's and say
  LGPL-3.0-or-later AND BSL-1.0.
- //tools:check expects the new header of Progmasoft files; the
  amalgamation notes say LGPL-3.0-or-later and the amalgamated files
  are regenerated; meson's project license is BSL-1.0 and
  LGPL-3.0-or-later.
- README gains a License section; LICENSES/README.md, contributing.md
  and progmasoft-catch3.md describe the new split and that releases
  before 0.5.0 keep the license they were published under.

Every commit to Progmasoft's files since the fork is the maintainer's.

Claude-Session: https://claude.ai/code/session_01Ji7xZMYjc4zankoc7hGBE1

1 files changed, 2 insertions, 2 deletions

diff --git a/tools/Checks.cpp b/tools/Checks.cpp
index a348eff4..473b740a 100644
--- a/tools/Checks.cpp
+++ b/tools/Checks.cpp
@@ -1,5 +1,5 @@
 // Copyright (c) 2026 Progmasoft.
-// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+// SPDX-License-Identifier: LGPL-3.0-or-later
 
 #include <algorithm>
 #include <set>
@@ -27,7 +27,7 @@ namespace Progmasoft::Catch3::Tools
 
         constexpr std::string_view kProgmasoftLicense
             = R"(// Copyright (c) 2026 Progmasoft.
-// SPDX-License-Identifier: MPL-2.0 WITH AdditionRef-Progmasoft-Exception-2.0
+// SPDX-License-Identifier: LGPL-3.0-or-later
 )";
 
         [[nodiscard]] bool