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 -->
+
+# Progmasoft C++ Style
+
+Progmasoft C++ Style is a style guide for C++20. It says how code is named, laid out, structured and checked, and why,
+so that a reader can move between files and projects without relearning the conventions each time.
+
+It is written for anyone who wants a complete, opinionated rule set to adopt. Progmasoft's own C++ projects follow it,
+among them the [Tala compiler](https://github.com/Progmasoft/tala) and [Catch3](https://github.com/Progmasoft/catch3).
+
+## How to read it
+
+Each rule is short and states what to do. Most come with the reason, because a rule whose reason is known can be
+applied to a case it did not foresee. Examples show the preferred form first; a form marked **Avoid** shows what the
+rule replaces.
+
+The words follow [RFC 2119](https://www.rfc-editor.org/rfc/rfc2119) in spirit: **must** is a rule a review enforces,
+**should** is the default that a good reason, written down next to the code, may override, and **may** is a choice
+left to the author.
+
+## Contents
+
+| Chapter | Covers |
+| --- | --- |
+| [1. Principles](Guide/PRINCIPLES.md) | what the guide optimizes for, and how to decide a case it does not cover |
+| [2. Formatting](Guide/FORMATTING.md) | the `.clang-format` profile, and what formatting leaves to the author |
+| [3. Naming](Guide/NAMING.md) | types, functions, variables, constants, namespaces, files, macros |
+| [4. Files and headers](Guide/FILES-AND-HEADERS.md) | extensions, `#pragma once`, includes, file size, the order inside a file |
+| [5. Language](Guide/LANGUAGE.md) | C++20 features: `auto`, initialization, `const`, casts, integers, templates, lambdas |
+| [6. Types and classes](Guide/TYPES-AND-CLASSES.md) | structs and classes, special members, inheritance, operators, enums |
+| [7. Errors](Guide/ERRORS.md) | exception-free code, `Expected` and `Error`, invariants, fatal defects |
+| [8. Ownership and memory](Guide/OWNERSHIP-AND-MEMORY.md) | RAII, smart pointers, views and their lifetimes, containers |
+| [9. Comments and documentation](Guide/COMMENTS-AND-DOCUMENTATION.md) | Doxygen, what a comment is for, TODOs |
+| [10. C interfaces](Guide/C-INTERFACES.md) | C11 headers, `extern "C"`, prefixes, ownership across the boundary |
+| [11. Concurrency](Guide/CONCURRENCY.md) | threads, locks, atomics, thread-safety annotations |
+| [12. Testing](Guide/TESTING.md) | unit tests, fuzzing, sanitizers, benchmarks |
+| [13. Tooling](Guide/TOOLING.md) | compiler flags, the `.clang-tidy` profile, CI checks |
+
+[Adopting the guide](ADOPTING.md) explains how a project points to this guide, which files it copies and how it
+records a deviation. [Contributing](CONTRIBUTING.md) explains how to propose a change to a rule.
+
+## The short version
+
+- C++20, formatted by the [clang-format profile](Guide/FORMATTING.md#the-profile), checked by the
+  [clang-tidy profile](Guide/TOOLING.md#132-the-clang-tidy-profile) with every warning an error.
+- `PascalCase` for types, functions and namespaces; `camelCase` for variables and parameters; `name_` for private data
+  members; `kPascalCase` for constants and enumerators; `UPPER_SNAKE_CASE` for macros.
+- Four spaces, braces on their own line, 80 columns, the return type on its own line.
+- `.hpp` and `.cpp`; `.h` only for a C interface. `#pragma once`. Standard and third-party headers in `<>`, the
+  project's own in `""`.
+- No `using namespace`. No exceptions in libraries and tools: recoverable failures return `Expected<T>` or `Error`.
+- Ownership is visible in the type: values and `std::unique_ptr` own, references and views borrow.
+- Every public declaration has a `///` comment, in US English.
+- Tests sit next to the code they test; sanitizers and fuzzers run in CI. The guide prescribes no test, fuzzing or
+  benchmark framework, and no error library: it prescribes how they are used.
+
+## License
+
+The text is licensed under [CC BY 4.0](LICENSE.md). The configuration files and code examples are dedicated to the
+public domain under [CC0 1.0](LICENSES/CC0-1.0.md), so they can be copied into any project without credit.