Tooling: generate compile_commands.json for clangd

The Visual Studio CMake generator does not emit compile_commands.json, so
clangd had no include paths or flags and reported hundreds of false errors.
Add a parallel Ninja config (build-clangd/) that produces the database:

- CMAKE_EXPORT_COMPILE_COMMANDS ON (honored by Ninja, ignored by VS)
- gen-compile-commands.bat configures build-clangd/ under vcvars64
- .clangd points clangd's CompilationDatabase at build-clangd/
- ignore /build-clangd

clangd's clang-cl driver auto-resolves the MSVC/Windows SDK system includes,
so the cl.exe-based database is sufficient.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CMakeLists.txt

@@ -13,6 +13,11 @@ set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
+# Emit compile_commands.json so clangd resolves includes/flags. Ignored by the
+# Visual Studio generator, so configure the Ninja build (build-clangd/) for this;
+# .clangd points clangd at that compilation database.
+set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
+
 # Single output dir keeps the donor-launch / inject workflow simple: host exe and
 # hook dll land next to each other under /bin (already in .gitignore).
 set(COOP_OUTPUT_DIR "${CMAKE_SOURCE_DIR}/bin/$<CONFIG>")