Write the first revision of Progmasoft C++ Style

Progmasoft C++ Style is a style guide for C++20, written for anyone
who wants a complete rule set to adopt. It writes down the conventions
the Progmasoft C++ projects already follow (naming, the clang-format
and clang-tidy profiles, exception-free error handling, Doxygen) and
decides what they left open.

Thirteen chapters under Guide/:

1. Principles: what the guide optimizes for and how to decide a case
   it does not cover.
2. Formatting: the full .clang-format profile, the reason for each
   choice, and the rules the tool cannot apply.
3. Naming: casing for every kind of name, acronyms, choosing words,
   namespaces, files.
4. Files and headers: extensions, #pragma once, includes, what goes in
   a header, the order inside a class, file size, vendored code.
5. Language: functions, auto, initialization, const and constexpr,
   casts, integers, strings, enumerations, templates and concepts,
   lambdas, control flow, and the features not used.
6. Types and classes: struct or class, final, construction, the rule of
   zero, inheritance, operators, strong types, optional and variant.
7. Errors: no exceptions by default, Expected and Error, invariants,
   fatal defects, destructors, diagnostics.
8. Ownership and memory: owners and borrowers, views and lifetimes,
   RAII, containers and deterministic iteration, allocation, undefined
   behavior.
9. Comments and documentation: Doxygen for public declarations,
   comments that explain why, TODOs that name an issue.
10. C interfaces: C11 headers, prefixes, opaque types, ownership and
    failure across the boundary, stability.
11. Concurrency: sharing less, threads, locks, thread-safety
    annotations, atomics, statics.
12. Testing: unit tests, fuzzing, sanitizers, benchmarks, coverage.
13. Tooling: the compiler flags, the full .clang-tidy profile with the
    reason for every disabled check, other checks, version pinning.

The guide prescribes no test, fuzzing or benchmark framework and no
error library; it prescribes how they are used. ADOPTING.md explains
how a project points to the guide, copies the profiles and records its
deviations. CONTRIBUTING.md explains how to propose a change.

The text is licensed under CC BY 4.0. The configuration and code
examples are dedicated to the public domain under CC0 1.0, so projects
can copy them under their own license.

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

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+<!-- SPDX-FileCopyrightText: 2026 Progmasoft <support@progmasoft.com> -->
+<!-- SPDX-License-Identifier: CC-BY-4.0 -->
+
+# Contributing
+
+Progmasoft C++ Style changes when a rule turns out to be wrong, unclear or missing. Proposals are welcome from anyone,
+whether or not they work on a Progmasoft project.
+
+## Proposing a change
+
+Open an issue first for anything beyond a correction of wording, a typo or a broken link. Describe:
+
+- **the rule**, as it reads now or as you would write it;
+- **the problem**: code that the current rule makes worse, a bug it fails to prevent, or a case it does not cover;
+- **the cost of the change** for projects that already follow the guide: how much code would have to change, and whether
+  a tool can do it.
+
+A rule earns its place by preventing a real class of mistakes or by removing a choice that would otherwise be argued
+about in every review. "Another style guide does it" is a starting point, not a reason.
+
+## Writing the change
+
+- Keep the shape of the guide: a rule in bold that says what to do, then the reason, then an example when it helps.
+- Examples follow every rule of the guide, including formatting by the profile and the 80-column limit.
+- A change to `.clang-format` or `.clang-tidy` says which clang version it was tested with and what it changes in
+  formatted code.
+- Write in US English, in the present tense and the active voice.
+- Note the change under `## Unreleased` in [CHANGELOG.md](CHANGELOG.md), saying whether existing code must change.
+
+## License of contributions
+
+By contributing you agree that your text is licensed under [CC BY 4.0](LICENSE.md) and that your configuration and code
+examples are dedicated to the public domain under [CC0 1.0](LICENSES/CC0-1.0.md), like the rest of the guide.