Replace MD5-based file creation tests with structural output validation

[ifilot]

Motivation

• MD5 checksums are fragile when switching architectures and introduce an external OpenSSL/test dependency, so file-validation should be resilient and architecture-independent.
• The goal is to validate generated OBJ/PLY/STL outputs by their structure and content counts rather than platform-specific hashes.

Description

• Rewrote TestFileCreation to remove MD5 hashing and instead generate a mesh reference and perform structural checks for each output format (src/test/test_file_creation.h, src/test/test_file_creation.cpp).
• Added a MeshReference type and per-format assertion helpers that validate: OBJ v/vn/f record counts, PLY header fields and binary payload size, and STL triangle count and expected binary size.
• Removed the OpenSSL/libcrypto test dependency and references by dropping pkg_check_modules(CRYPTO ...) and PkgConfig::CRYPTO from src/test/CMakeLists.txt, and removed OpenSSL from the installation prerequisites in docs/installation.rst.
• Replaced MD5-specific headers and code (<openssl/...>, MD5 helper) with standard includes (<cstdint>, <filesystem>, <string>) and clearer file-shape assertions.

Testing

• Verified no remaining MD5/OpenSSL/Crypto references with a repository search using rg which returned no matches in the updated test/docs files (success).
• Attempted a CMake configuration with cmake -S src -B build as an automated build check, which could not complete in this environment because Boost development components (iostreams, filesystem) were not available (configuration failed).

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Codex Task