Modernize file I/O with RAII patterns in throughput_estimator

[Copilot]

Addresses code review feedback on PR #808 requesting modernization of C-style file I/O to use RAII patterns and reduced nesting.

Changes

• Replaced FILE* with std::ofstream

  • DownloadContext class now uses std::ofstream member instead of raw pointer
  • Automatic resource cleanup via destructor - no manual fclose() needed
  • All fprintf() calls replaced with stream insertion operators

• Flattened control flow in main.cpp

  • Early returns on error conditions eliminate 4 levels of nesting
  • Error messages now use std::cerr instead of fprintf(stderr)

• Exception safety

  • Files automatically close on scope exit, even during exceptions
  • No resource leaks in error paths

Example

Before:

FILE *f_ABR = fopen(pathABR.c_str(), "wb");
if (!f_ABR) {
    fprintf(stderr, "unable to open %s\n", pathABR.c_str());
} else {
    // 3 more levels of nesting...
    fprintf(f_ABR, "%f,%f\n", value1, value2);
    fclose(f_ABR);
}

After:

std::ofstream f_ABR(pathABR, std::ios::binary);
if (!f_ABR.is_open()) {
    std::cerr << "unable to open " << pathABR << "\n";
    return EXIT_FAILURE;
}
f_ABR << value1 << "," << value2 << "\n";
// Automatic cleanup on scope exit

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