C++26 support: build flags, headers, parser features

Build:
- CppBuild.java: -std=c++20 -> -std=c++2c (GCC C++26 draft flag)
- generate_plugandplay.py: -std=c++17 -> -std=c++2c throughout
- generate_cmake.py: CMAKE_CXX_STANDARD 17 -> 26

Processing.h:
- C++23 headers: <expected>, <flat_map>, <flat_set>, <generator>,
  <print>, <spanstream>, <stacktrace> guarded by __cplusplus >= 202302L
- C++26 headers: <inplace_vector>, <function_ref>, <hazard_pointer>
  guarded by __has_include and __cplusplus > 202302L

Parser:
- if consteval: C++23 immediate-function context check now parsed;
  emitted as if constexpr(true) for runtime compatibility
- = delete("reason"): C++26 optional string message on deleted
  functions now consumed

CppBuild:
- C++23/26 using declarations added to generated preamble:
  std::expected, std::unexpected, std::print, std::println,
  std::inplace_vector (all guarded by __cplusplus version checks)

Author: pepc84

dist/processing-cpp-cmake.zip

@@ -297,7 +297,7 @@ def generate(out_dir: Path) -> None:
 # scripts/generate_cmake.py. Don't hand-edit a copy of this folder;
 # re-run that script instead.
 
-set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD 26)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 # Needed early -- CMAKE_INSTALL_INCLUDEDIR (used a few lines down, in
@@ -321,7 +321,7 @@ def generate(out_dir: Path) -> None:
     PROCESSING_HAS_STB_TRUETYPE
 )
 
-target_compile_features(processing_cpp PUBLIC cxx_std_17)
+target_compile_features(processing_cpp PUBLIC cxx_std_26)
 
 option(PROCESSING_CPP_FETCH_DEPS "Build GLFW/GLEW from source if missing" OFF)