C++ ergonomics

[treeform]

• Use C++ standard integer spelling in generated C++ headers: <cstdint>, std::int*_t, std::uint*_t, std::intptr_t, and std::uintptr_t.
• Emit C++ constants as static constexpr values and enum aliases as using ... = std::uint8_t with typed constexpr entries instead of preprocessor macros.
• Store generated C++ reference handles as std::uintptr_t and return bool from generated C++ equality ABI declarations.
• Add C++ sequence wrapper constructors and methods: size, get, operator[], set, removeAt, add, and clear.
• Add owner methods for exported sequence fields, such as addGradientStops, gradientStopsSize, and getGradientStops.