Fix #95: Add README_design.md documenting template design choices

New top-level doc explains the rationale behind the template's CMake
layout, C++23 default, warning/sanitizer/hardening flags, CPM-based
dependencies, dual top-level/subdir behavior, and the
myproject_ENABLE_* knobs. Wording is kept short and specific so
beginners and veterans can adapt the defaults knowingly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: leftibot

README_design.md

@@ -0,0 +1,98 @@
+# Design Choices
+
+This template starts a C++ project with safe, modern defaults. Each choice
+below explains *why*, so you can keep it, swap it, or turn it off.
+
+## Goals
+
+1. Catch bugs at compile time, not in production.
+2. Stay portable across GCC, Clang, MSVC, and Emscripten.
+3. Work the same way as a top-level project or as a subdirectory dependency.
+
+## Layout
+
+| File | Role |
+| --- | --- |
+| `CMakeLists.txt` | Top-level wiring. |
+| `ProjectOptions.cmake` | All `myproject_*` options and setup macros. |
+| `Dependencies.cmake` | CPM package fetch, gated by `if(NOT TARGET ...)`. |
+| `cmake/*.cmake` | One concern per file (warnings, sanitizers, hardening, ...). |
+
+`PROJECT_IS_TOP_LEVEL` flips defaults: strict when you own the build, quiet
+when you are a dependency.
+
+## C++ standard
+
+C++23, set only if a parent project has not chosen one. `CMAKE_CXX_EXTENSIONS`
+is off so the standard flag is `-std=c++23`, not `-std=gnu++23`. This avoids
+`-Wpedantic` conflicts with precompiled headers.
+
+## Warnings
+
+`cmake/CompilerWarnings.cmake` enables a curated set per compiler — `/W4`
+plus extras on MSVC, and `-Wall -Wextra -Wshadow -Wconversion -Wpedantic ...`
+on GCC/Clang. Top-level builds add `-Werror` / `/WX`. Source:
+[cppbestpractices](https://github.com/lefticus/cppbestpractices/blob/master/02-Use_the_Tools_Available.md).
+
+## Sanitizers
+
+ASan and UBSan are on by default for top-level GCC/Clang builds when a link
+probe shows them working. TSan, LSan, and MSan are off — they conflict with
+each other and MSan needs an instrumented standard library. Emscripten and
+MSVC skip the sanitizer pass.
+
+## Hardening
+
+`cmake/Hardening.cmake` adds `_FORTIFY_SOURCE=3` (release builds),
+`_GLIBCXX_ASSERTIONS`, `-fstack-protector-strong`, `-fcf-protection`, and
+`-fstack-clash-protection` when supported. MSVC gets `/sdl /DYNAMICBASE
+/guard:cf /NXCOMPAT /CETCOMPAT`. When no full sanitizer is active, the UBSan
+minimal runtime is layered on top.
+
+## Static analysis
+
+clang-tidy and cppcheck run as part of the build, on by default at top level.
+They are separate options because one tool may not be installed in every
+environment.
+
+## Link-time optimization
+
+IPO/LTO is on by default at top level. It is gated through
+`check_ipo_supported` so unsupported toolchains skip it.
+
+## Dependencies
+
+[CPM](https://github.com/cpm-cmake/CPM.cmake) fetches sources at configure
+time. Each package is gated by `if(NOT TARGET ...)`, so a parent project can
+supply its own version. `SYSTEM YES` silences warnings from third-party
+headers. Default set: fmt, spdlog, Catch2, CLI11, FTXUI, lefticus/tools.
+
+## Testing
+
+* `test/tests.cpp` — Catch2 unit tests.
+* `test/constexpr_tests.cpp` — the same checks at compile time, so bugs
+  become build errors.
+* `fuzz_test/` — libFuzzer harness, auto-enabled when ASan/TSan/UBSan and
+  libFuzzer are all available.
+
+## Targets and packaging
+
+`myproject_options` and `myproject_warnings` are `INTERFACE` libraries that
+hold flags. Real targets link them to inherit the configuration without
+touching global state. `CPack` package names embed compiler, version, and
+short Git SHA, so a binary maps to one build.
+
+## Defaults for daily use
+
+The default build type is `RelWithDebInfo` — debuggable and fast.
+`compile_commands.json` is always exported, for editors and clang tooling.
+
+## Changing the defaults
+
+Every knob is a CMake option named `myproject_ENABLE_<feature>`. Flip it on
+the configure line, for example:
+
+    cmake -B build -S . -Dmyproject_ENABLE_CLANG_TIDY=OFF
+
+The `myproject_` prefix is the placeholder the rename workflow replaces, so
+renaming the project is one search-and-replace.