Point the Progmasoft-authored code at Progmasoft C++ Style

The contributing guide described only the style of the code derived
from Catch2. The code under src/Progmasoft/ and tests/Progmasoft/ is
not derived from it and follows Progmasoft C++ Style
(https://github.com/Progmasoft/progmasoft-cpp), the C++ guide of the
other Progmasoft projects.

A new section under "Writing code" says so, keeps the upstream style
for the Catch2-derived code so upstream changes still apply cleanly,
and records the two deviations the Catch2 API requires: the namespace
Progmasoft::Catch3, and exceptions, which report a failed assertion and
stay inside the library.

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

1 files changed, 19 insertions, 0 deletions

diff --git a/docs/contributing.md b/docs/contributing.md
index 30254416..e67ea237 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -243,6 +243,25 @@ the size of your diffs reasonable, you should only use clang-format
 on the newly changed code.
 
 
+### Progmasoft-authored code
+
+The code under `src/Progmasoft/` and `tests/Progmasoft/` is not derived
+from Catch2 and follows
+[Progmasoft C++ Style](https://github.com/Progmasoft/progmasoft-cpp):
+its naming, its formatting profile, its language and documentation
+rules. The code derived from Catch2 keeps the upstream style described
+on this page, so that changes from upstream still apply cleanly.
+
+Progmasoft-authored code deviates from the guide where the Catch2 API
+requires it:
+
+* It lives in the namespace `Progmasoft::Catch3`, with helpers in
+  `Progmasoft::Catch3::Detail`.
+* It may throw, because a failed assertion is reported by an exception,
+  as everywhere in Catch2. Exceptions stay inside the library and never
+  cross into the code under test.
+
+
 ### Code constructs to watch out for
 
 This section is a (sadly incomplete) listing of various constructs that