Restore strict selective member retrieval

[unitmatrix]

Summary

• reject servers that ignore required HTTP Range requests instead of downloading the complete archive
• remove the complete-download and local-file fallback path
• replace fallback-success coverage with extraction and listing rejection coverage that proves every request remains ranged
• synchronize README and man-page behavior

Verification

• GCC build and tests: 8/8 passed
• Clang build and tests: 8/8 passed
• ASan and UBSan build and tests: 8/8 passed with platform-supported strict settings
• installed static and shared CMake package consumers passed