Fix C++20 compatibility: replace deprecated std::result_of and fix std::max type mismatch

[Copilot]

This PR fixes two critical C++20 compatibility issues that prevent the project from building with modern C++ standards:

Issues Fixed

1. Deprecated std::result_of Usage

The std::result_of type trait was deprecated in C++14 and removed in C++17. The code was using it in the ThreadPool template implementation:

// Before (deprecated)
template<typename F>
auto enqueue(F&& f) -> std::future<typename std::result_of<F()>::type>;

// After (C++20 compatible)
template<typename F>
auto enqueue(F&& f) -> std::future<typename std::invoke_result_t<F>>;

2. Type Mismatch in std::max

The std::max function requires both arguments to be the same type. The code was mixing Lamports (uint64_t) with unsigned long:

// Before (type mismatch)
nonce_account.lamports = std::max(nonce_account.lamports, 1447680UL);

// After (types match)
nonce_account.lamports = std::max(nonce_account.lamports, static_cast<Lamports>(1447680UL));

Changes Made

Files Modified:

• include/svm/enhanced_engine.h: Updated ThreadPool template declaration
• src/svm/enhanced_engine.cpp: Updated ThreadPool template implementation (2 locations)
• src/svm/spl_programs.cpp: Fixed std::max type casting in nonce account initialization

Verification

• ✅ All 16 test suites pass (100% success rate)
• ✅ Clean build with C++20 standard
• ✅ No remaining deprecated API usage
• ✅ Verified with strict compiler warnings (-Wall -Wextra -Werror)

These minimal surgical changes ensure the codebase compiles with modern C++ standards while preserving all existing functionality and performance characteristics.

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